Only Wither Skeletons drop their skull. I think that's a bug, i don't understand why creeper/zombie/skeleton heads shouldn't be obtainable in survival gamemode.
What I expected to happen was...:
At least 1 on 200 zombies drops its head
What actually happened was...:
Only 64*3+35 rotten flesh, 4 iron ingots, 2 potatoes, 1 carrot, leather pants, gold boots
How to reproduce:
Kill a lot of skeletons/zombies/creepers: no heads will be dropped.
MC-2326
Only Wither Skeletons drop their head
Resolved
Duplicate
2
(Unassigned)
Unconfirmed
creeper, drop, head, skeleton, skull, zombie
Minecraft 1.4.2, Minecraft 1.4.3
Retrieved
This is not a bug. At present, only wither skeletons have head drops in game. All other mob heads can only be obtained in creative mode.