Quote from FreelanceZero
Funny bug we encountered. Me 'n my friends started up a Minecraft server with a custom modpack, but because of some issues we ran into, we decided to remove MoCreatures.
After we removed the mod and booted the server back up, there were no mobs. No spiders, zombies, skeletons, and creepers. No sheep, chicken, pigs, and cows. None at all. Nada.
We also checked the Nether. Again, absolutely nothing. It was and still is downright creepy. Monster Spawners in dungeons work A-OK, though.
Note that this server was not one I myself hosted, but one my friend hosted. We used a custom modpack, so it could be that it's merely mods conflicting with one another initially, and now it's just messing up the files, but I think this is a bug worth looking into.
I mentioned the same issue a couple pages back, though I was so focused on hostile mobs that I didn't notice the others missing, mind comparing mod lists?