Many times I have been about to mount a horse only to have it struck out from under me by my own team, which sometimes causes me damage too! Another thing I RAGE about is spawning on horseback, only to have my own team mates strike my horse and sometimes me. If you guys are so enraged that you need to strike something, go for the enemy!
If I see an enemy going for a horse I will try to call it away from them, or kill it if the enemy was a rider just unhorsed.
The horses are valuable to your teams riders, so take this into consideration when you are just rage killing them.