Affects Version/s: Minecraft 1.13
Fix Version/s: None
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.
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)