Mobs & Projectiles

When a skeleton, stray, or other mob that attacks using projectiles is riding something, their projectiles can hit whatever they're riding...unless it's a horse, because there's a specific exception for horses thanks to the Four Horsemen.

Preventing mobs from hurting their own mounts with projectiles, regardless of what that mount is, would broaden possibilities for mapmakers without negatively impacting normal gameplay, make intuitive sense, and potentially open future options for things like Pillagers riding Ravagers.