If using horseback to attack mobs - if attacked by mob, horse cannot be invincable... Either joint hp or separate hp, i'm not sure exactly what. But if you are able to ride a horse then to make it realistic, when the mob atacks you back, both you and the horse need to recieve dmg and die..
suugestion: If horses are able to be ridden horseback style, allow other mobs to be "riden" too. Just create new equipment - saddle. new requirements, to be able to put a saddle onto mob. idk- 1.tamed. 2. certain skills above so and so. if the pet has sufficient hp + skills/avoid skills, then it should be able to stay alive by the time you kill the mob and then you can heal your pet afterwards. - basically, this would be a way to move faster, but also hunt with advantage. (btw, (i'm not complaining at all) but adding transportation will render the speed/flex skills less valueable. Perhaps have terrain where vehicles are able to access, terrain where pets are able to access, and terrain where only able to access on foot?