I think that the mounts have to give bonus to the player instead of being just other creatues. Like more hit points, shield bonus, attack bonus. They could improve your weapon or shield, give you health each turn as the SoG does, take over the damage (as a gravity pull) etc.

mount: give momentum to your weapon and has a gravity pull on it

give inmortality to your shield

flying weapon

steal opponent weapon and/or shield

poison to the weapon

protect artifact to both shield and weapon

antimatter to opponet weapon

fire damage to your weapon or fire shield bonus to your shield

hp recovering and/or purify

up hp maximum

stasis to enemy weapon on each attack

congeal ability to your shield