Board Thread:Suggestions forum/@comment-25981649-20170817085134/@comment-25981649-20170817165750

LordArvedui wrote: Though it really shouldn't be immune to arrows, I don't believe Tolkien ever said they were and that would be very unbalanced. "Killing a  mûmak was almost impossible - its rough, leathery hide made arrows relatively harmless, and any archer standing to take a shot would be an easy target for the archers on top of the  mûmak."

This is from the Tolkien Gateway, they cited a source- you can go check it out on the website.

and @MinecraftImage113

Yep!