Using any spawn egg on an untamed horse/donkey/llama except for its own causes the player to start riding it

How to reproduce

1. Use a horse spawn egg to spawn a horse

2. Right-click on it with a normal item (horse rejects it)
3. Right-click on it with a creeper spawn egg (horse is mounted)

Applies to any spawn egg, except for the horse spawn egg itself (that will spawn a baby as expected).

Affects horses, donkeys, llamas, etc. Does not affect saddled pigs.