I was aiming at a monster and my horse behind me somehow got hit instead.

I was in my hardcore world at nighttime when I encountered some monsters, and I decided to use my high-powered enchanted bow on I think a drowned that was carrying a trident when for some reason my horse that was off camera got hit instead leaving it's hp to 2 hearts/4 hp.

Steps to reproduce:
1. Make a bow and at least 1 arrow
2. Spawn or find a mob
3. Make that mob be behind you and close to you
4. Fire the arrow
5. The mob gets hit

This might work for other ranged weapons too.