When shooting an arrow while riding a horse, the horse should take damage rather than the arrow going the horse and nothing happening.
How to reproduce
Start by obtaining a bow and some arrows, find a horse and tame it
Ride the horse and shoot an arrow through the horse while riding it, nothing will happen
Observations: Arrows can go through horse while riding it.
Expectations: It should take damage.
This is WAI according to MC-176195. Mounts aren't meant to be hit by projectiles shot by the rider.