Using a horse spawn egg will occasionally spawn a baby horse rather than an adult horse, regardless of whether it was spawned on another horse. This seems to only happen with horses, not other animals.

To Reproduce:

Step 1: Get a horse egg

Step 2: Use horse egg on ground (not on another horse)

Step 3: Notice that a baby horse has been spawned (it seems to happen about 1/10 times so keep trying until the bug occurs)