<p>You can unpack the mod file as a zip (.jar is just a .zip with a different extension to make it obvious what it is). There's a bunch of files in there, you can find the map and change it.
</p><p>Idk about spiders vs horses, you could check the
wiki pages for each, I bet the average speed values are there. It's also possible that they're defined in the mod file someplace, but you'll need a java decompiler to read them.
</p>
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