Occasionally mobs will disappear. Mobs that I have witnessed this bug affecting are Cows, Zombies and Skeletons. I am unsure if this bug effects any other mobs.
MC-84667
Disappearing Mobs
Resolved
Duplicate
0
George
(Unassigned)
Unconfirmed
Minecraft 15w31c
Retrieved
Does MC-65040 describe your issue?