When you right click a horse, donkey or mule with its spawn egg while aiming at the upper part of the hitbox you mount on the mobs and spawn the baby variant. And the hand swing animation also does not play because of MC-180922.

How to reproduce:

1. Spawn a horse.
2. Aim at the upper part of the horse.
3. Right click the horse with the spawn egg.

You will mount on the horse and spawn the baby variant. It also works with donkeys and mules.