What I expected to happen was the horse that I spawned with a spawn egg would take damage on the first hit.
What actually happened was the horse I spawned with a spawn egg did not take any damage unless if you hit it again.
The same bug applies to llamas, zombie horses, skeleton horses, donkeys, and mules as well.
Steps to Reproduce:
1. Spawn a horse, a donkey, or a mule via spawn egg.
2. Punch or hit it with a weapon once.
3. Mount it to see if it took the damage.