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