I made a minecraft world (messing around with Custom Worlds), and noticed this when I made a villager spawner.

The Textures were all messed up. Up to that point, all the mobs I had seen had normal textures (cows, pigs, sheep, zombies, etc.). I messed with the settings, and when I turned off alternate textures, they (bugged mobs) dissapeared (second to last screenshot).

Steps to Reproduce;
1. Start up Minecraft 1.8
2. Launch a Minecraft world
3. Spawn Mobs with Spawn Eggs
4. See the messed up textures