Seriously needs to be fixed.
The horse's head is so small, arrows pass through and into a shoulder or shield. Weapons pass through the head all the time. Can't count how many times I've watched my weapon go through the horses nostrils, only to get bumped and yes, slashed. Your horse should be dead!
The ass end of the horse is truncated apparently and only exists directly under the saddle itself. Extend that out to where the graphics end.
The width of the box is wider than the graphics so you get an invisible "shoulder bump" when you shouldn't.
Revisit, or post the hitboxes as they exist and prove me wrong.
Also, this would explain why horses suddenly no longer rear except maybe 10% of the time when hit from the front with a polearm.