Animal Spawning?!

Put the summary of the bug you're having here

What I expected to happen was...:
That when I kill my animals, new animals will respawn.

What actually happened was...:
There are now animals in my world other than chickens from eggs.

Steps to Reproduce:
1. KILL ANIMALS!
2. Turn it to peaceful mode.
3. Look around the whole entire world.
4. No animals