Minecraft wiki entry for Development_versions, associated text: Squid
Version exclusive: Baby squid
Version exclusive: Produces a black cloud of "ink" when attacked.
Version exclusive: Spawn only in ocean biomes.
Spider Jockey
Cave Spider
Version exclusive: Cave spider jockeys.
Bat
Baby Zombie
Baby zombies have a 5% chance of spawning as a chicken jockey. (In turn, zombies have a 0.2 percent chance of being a chicken jockey)
Magma Cube
Drop magma cream
Only spawn from spawn eggs.
Ghast
Only available through spawn eggs.
Spawn egg can only be received through inventory editing.