Probably better if Sampson is just a saddlehorse, not trained for fighting. A bonus to Evade a fight entirely if mounted seems reasonable to me. And a faster movement rate on maps if mounted to make bypassing/outrunning attackers easier. And, if the MC dismounts before a fight happens it's not included in the fight at all, he has time to tie it up or something.Combat might just be like normal & not consider the horse? Unless u get like Agility bonuses? That would be something Tinkerer would need to look into.
Hm. In a fight that isn't Evaded Sampson could always take Guard actions, the damage reduction de facto representing the higher HP he would have. The MC could have an increased Attack and lowered Defense, the "high ground" advantage vs reduced mobility/dodge & weave for sitting on something he can't completely control.