Edit: Duplicate of MC-65040.

If you go far enough away from a mob that it is out of render distance and then come back it does not render. It is fairly difficult to reproduce it but once you find the right distance you have to go away the bug is 100% consistent. It works best for me with render distance 3. And I was also unable to make mobs become invisible out of spawn chunks.

The mobs still interact and you can hear their sound. A+F3 does not solve the problem.

Attached images:

n1: Spawning a horse.
n2: Horse is invisible after coming back.
n3: Spawn a cow.
n4: Cow is invisible after coming back. Note: the horse in the background remains visible, which is not always the case.
n5: Sometimes leads knot does not render, sometimes it does but the lead itself doesn't.
n6: Spawning a zombie.
n7: Zombie is invisible after coming back but it still makes noise.
n8: Attacked by an invisible zombie.
n9: After a few seconds the zombie reappears.