These are some minor and somewhat major mobs that I would like to see eventually added to the mod. I found all this information on Tolkien Gateway:
Kirinki: This was one of the many species of birds found in Numenor, and doubtless Amen, that were not known in Middle-Earth. The kirinki were said to be smaller than wrens, with bright scarlet feathers, and voices so high that they could hardly be heard by Men. I think that the Kirinki should spawn exclusively in Meneltarna
Gorcrows: I know that these birds are already in the mod, but I have a few suggestions. They should croak in their sleep, and the Long Marshes should have a tiny subbiome called the marsh of Tode. Gorcrows should spawn there instead of the Long Marshes itself. Gorcrows are said to have a 4 ft. wingspan and to be smaller cousins of the crebain, and to be attracted to shiny things. They should also spawn in the Lone-lands and appear frequently in Mordor. There should also be a variant called the Northern Gorcrow, spawning within Forodwaith. This variant should be a larger version of the common gorcrow. The Lossoth are said to regard the northern gorcrows as birds of ill-omen. My suggestion is that they could attack similarly to Minecraft Phantoms, swooping down to attack before flying back up again.
Pards: These were feline creatures. Apparently the pards lived in wooded lands of the East (perhaps referring to the lands beyond Rhun, the Uttermost East). The beast, "fleet upon feet", leaped on its prey from above. At the Bodleian Library in Oxford are kept several manuscripts of medieval bestiaries containing descriptions of the legendary pard. The animal is described as very swift, killing its prey with a single leap. It was believed that the mating of a pard and a lion produced the leopard. In 1962 Tolkien published the poem “Cat,” which, carrying many similarities with this medieval notion, shows that he was likely familiar with these manuscripts.
Lintips: were mysterious creatures only mentioned in verse. They appear at night under the moon; Tom wondered whether they slide down from the Moon, or come in star-winks. Apparently they are very small, and smell mousy.
Tom noticed them from their smell, one night they came too early; he told them to drink up the dew from the grass, but minding his feet. They just ran away and laughed at him to Tom's dismay who wished them to stay, leaving him complaining that they are the only things that won't talk to him, and wondered if they hide something.
Hummerhorns: These were a race of winged insects. The hummerhorns are described as aggressive creatures, weighing 6-8 lbs and resembling gigantic black and grey wasps. They probably should spawn in the Midgewater Marshes. They should also spawn as a variant of the Minecraft Bee.
Dumbledors: These are described as being large winged insects weighing 6-8 pounds, carrying illness or poison, and to live in the Midgewater Marshes. Because they are another word for Bees, they should spawn as a variant of the Minecraft Bee.
The Swans of Gorbelgod: These were a race of Swans remembered in legend on Numenor. They were white and of an unusually great size. Little else is known about these creatures, or about the location of Gorbelgod. In a manuscript, J.R.R. Tolkien used the name Dur nor-Belgoth and changed it later to Gorbelgod. I think that these should maybe spawn alongside the Kirinki.
Turtle-fish: These were a legendary race of giant sea monsters. The last of the Turtle-fish was named Fastitocalon. Turtle-fish appeared as treacherous islands, really being monsters capable of drowning seafarers. It is never explained whether the turtle-fish was an actual race or just a fictional character for the poem "Fastitocalon". In an earlier version of the legendarium, Ulmo, Lord of the Waters, uses his giant whale, Uin, to drag Tol Eressea towards Aman. This whale reappears in J.R.R. Tolkien’s story “Roverandom.” It is thinkable that Tolkien, in an early phase of the mythology, imagined the Fastitocalon to be another one of Ulmo's creatures. The Turtle-fish could maybe spawn in Belegar. These creatures are what I really most want to see in the mod. Even if they are added in the form of lore books.
Were-worms: These were creatures of an unknown kind, possibly mythical and presumably related to Dragons, that were said to dwell in the Last Desert. Thus Were-worms, if they did exist, could be a man-like type of Dragon, though this must remain in the realm of speculation.
Sea-serpents: These were also called the Fish-dragons, were a breed of dragons only mentioned fleetingly in Elvish linguistic writings. No story tells of their appearance in the history of Arda. Water Drakes, or Water Serpents, are drakes who have adapted to a life in freshwater pools or in the ocean. The smaller varieties, who only live in fresh-water, are called Rain-drakes, or Lake-worms, while the larger, who prey in all types of water, are called True Water-drakes or Sea Serpents. An individual Water-drake, Séahmatha, or Lake Serpent in the tongue of the Northmen, inhabits the Long Lake.
Great Beasts: were used to pull the battering ram Grond during the siege of Minas Tirith. Whether these beasts of burden used by Sauron were a foul breed of Mumakil, or a race of unknown animals from distant lands, is impossible to say for certain as we lack a more detailed description of the creatures. In The Lord of The Rings Online, the great beasts are depicted as warty, multi-horned rhinoceros-like creatures known as "Sûlokil". They can be found on the Plateau of Gorgoroth.
Glowworms: These were tiny animals which glimmered in dark environments. Glowworms are also said to have been creeping about the borders of the deep pools of Murmuran. Maybe when the mod gets updated to 1.17, these Glowworms could spawn in the Deepslate layer.
Great Glow-worms: are said to have crept about the grassy borders of Olore Malle, the Path of Dreams. As these creatures are mentioned only fleetingly, it is impossible to say if they were related to wingless dragons or if they were giant versions of regular Glowworms. One can assume however that they glowed in the dark. Maybe when the mod gets updated to 1.17, these creatures could spawn in the Deepslate layer.
Spark-dragons: These were a wingless breed of dragons of whom all that is known is from the nomenclature by the Eldar. No story tells of their appearance in the history of Arda, but one can assume that somehow they radiated light. They prefer to fight with their tails, which hold an electrically charged fluid that delivers a sting analogous to the impact of a lightning bolt. Maybe when the mod gets updated to 1.17, these creatures could spawn in the Deepslate layer. There could also be an Utumno variant spawning in the fire layer of Utumno.
Neekerbreekers: These were large and loud insects related to crickets. They dwelt in the Midgewater Marshes and squeaked all through the night, calling "neek-breek, breek-neek.” Samwise Gamgee gave them the name Neekerbreekers. Neekerbreekers were named so because of their sound, perhaps like that of a cricket. These actually exist in the lore, so they should be added.
Ogres: These were monstrous and destructive creatures of legend and folklore of Middle-Earth, although it cannot be said if they indeed existed.
During the Riddle-game with Gollum, Bilbo Baggins, in his attempt to solve Gollum's fifth riddle, sat thinking of all the horrible names of all the giants and ogres he had ever heard told of in tales. No ogre is ever mentioned in the annals of the Elder Days and none played any role in the wars and battles. It is entirely possible that they were a mythical race.
It is also possible that Ogre was just another name for the Trolls. They are mentioned as cannibal Giants. Those of the north were known as Hongwir while the southern ones were named Sarqindi. Sarqindi is glossed as "Cannibal-ogres", and hongwir might be related to the Qenya word ongwe, meaning pain. Tolkien mentioned again ogres out of context in analyses of the name Orc: "...the gloss Þyrs ogre...,” and of the name Ettendales: "This is meant to be a Common Speech name, though it contains an obsolete element eten’troll, ogre’.”
Hobgoblin: This was a name “for the larger kinds" of Orcs found in Middle-Earth within the Third Age. In the quest for Erebor, Gandalf warns warns Bilbo Baggins that the Grey Mountains are "simply stiff with goblins, hobgoblins, and orcs of the worst description". Hobgoblins are a breed of large, tusked Orcs found in the Grey Mountains.
Gongs: These are evil beings that appear in “The Book of Lost Tales,” the earliest version of what would become The Silmarillion. In his commentary, Christopher Tolkien remarks that they are obscurely related to Orcs, perhaps as a sub-race of the species. There are no references to Gongs in J.R.R. Tolkien’s later legendarium. Mim commanded a guard of Orcs and Gongs to protect the treasures of Glorund in Nargothrond. They were slain by Hurin and his companions. The Gnomish word sithagong is glossed as dragonfly, with sitha being the word for fly.
Mewlips: These were a legendary race of evil creatures only mentioned in verse. The creatures were likely fictitious, although they could have been inspired by a faction of Orcs. Living beyond the Merlock Mountains and in the marsh of Tode, they are said to have fed on passers-by, collecting their bones in a sack. In “The Hobbit, Bilbo Baggins learned that the Long Marshes had grown wider, consuming paths and lost people. In plot notes that J.R.R. Tolkien made while writing the story, the raftsmen told tales of disappearing rafts, men, and beasts; the more ominous line in the notes may indicate that Tolkien had some unpleasantness in mind had Thorin and Company gotten into the marshes - perhaps meeting the Mewlips. It is likely that maybe the marsh of Tode in the verse refers to the Long Marshes and the Merlock Mountains refers to either the Misty Mountains or the Mirkwood Mountains. Maybe the Mewlips could be a derogatory term the dwarves called the Wood Elves. However, in my opinion, they should appear in the mod as an evil faction in the Long Marshes that inspire the verse.
White Wolves: These were a type of wolf native to the frozen lands in the north of Middle-Earth. They were not normally seen in Eriador, but seem to have remained among the snows of the Forodwaith and perhaps beyond. In the year T.A. 2911, one of the harshest winters in Middle-earth's history descended, the so-called Fell Winter. The ice and snow of the north came at least as far south as the Shire. With them came the White Wolves, invading Eriador and roaming as far as the Shire. The frozen Brandywine River meant that they were even able to cross into the Eastfarthing. This was the last time the Horn-call of Buckland was used before the War of the Ring, one hundred and seven years later. If these wolves are added to the mod, they should spawn in Forochel, the Angmar Mountains, and Forodwaith.
Werewolves: These were servants of Sauron, bred in the Elder Days from wolves and inhabited by dreadful spirits that Sauron had imprisoned in their bodies. Werewolves were created, or a least corrupted from some other form, by Sauron, who was their master, and who took the shape of a great wolf himself at least once. The first werewolf was Draugluin, and the greatest was Carcharoth, the guardian of Angband, a descendant of Draugluin as all other werewolves were. In the “Grey Annals,” it is told that "creatures that walked in wolf-shapes," entered Beleriand in the Valian Year 1330. Although not appearing as such in the known written records of Arda during the Second and Third Ages, Gandalf mentioned the werewolves as being among Sauron's servants in the late Third Age, along with orcs, trolls, wargs, and wraiths. In Sindarin, werewolves were called gaurhoth. The Quenya name was nauro. They cannot transform themselves into Men, although they can alter shape into being able to stand upright while remaining Wolf-like. Due to Werewolves being absent in the Third Age, maybe when the mod gets updated to 1.17, these creatures could spawn in the Deepslate layer. They could spawn in Werewolf Dens scattered throughout the Deepslate layer. There could also be an Utumno variant spawning in the Obsidian layer of Utumno.
Vampires: These were mysterious bat-like creatures in the service of Morgoth and of Sauron. The only vampire whose name is recorded in the annals of Arda is Sauron’s servant Thuringwethil, but Sauron himself took the form of a vampire on at least one occasion, to flee Huan. Vampires are described as former Maia patrons of normal bats, but corrupted by Morgoth and transformed into man-sized bats with the faces of hideous Men or Women, having the ability to change shape. A related creature is also envisioned, the Blood-wight, which is an undead Mannish shapechanger, draining blood from living beings. Due to Vampires being absent in the Third Age, maybe when the mod gets updated to 1.17, these creatures could spawn in the Deepslate layer. They could spawn in Vampire Nests scattered throughout the Deepslate layer. There could also be an Utumno variant spawning in the Obsidian layer of Utumno.
Goblins: I know that Goblin is used interchangeably with Orcs, but I propose that the Orcs of Moria and Goblin Town should be renamed to Goblins. Also Orcs of the Grey Mountains should be renamed to Hobgoblins.
Badger-Folk: The Badger-folk lived in secret houses connected by tunnels under a hill in, or nearby, the Old Forest. They appear to have been sentient and capable of speaking. Although Badger-brock once forcefully brought down Tom Bombadil to their burrows, the Badger-folk do not appear as truly evil creatures, but perhaps rather cunning or tricky.
Gostir: This creature was one of the Dragons of Morgoth about whom almost nothing is known except its name. Gostir means dread glance in Noldorin, from gost, which means dread and terror; and thîr, which means to look, face, expression, countenance. The older form of the name was Gorsthir. I think that maybe Gostir could be added to the mod as a boss. He could spawn in a generated structure called Gostir’s Lair in Forodwaith. Defeating him would provide the coordinates of Utumno.
I think that all of these ideas should be added in some form or another in the mod eventually.