Clementine the Mountable

Keen Hearing and Smell. The mastiff has advantage on Wisdom (Perception) checks that rely on hearing or smell.

Good Girl. Clementine has advantage on attack rolls against a creature if she is being ridden by her owner and said owner isn't incapacitated.

Soul Bonded. Add proficiency bonus of owner to hit point maximum and AC

Lead Belly. Can consume substances that would be toxic to most humanoids

Bite. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 4 (2d6 + 1) piercing damage. If the target is a creature, it must succeed on a DC 11 Strength saving throw or be knocked prone.

Clamp. Upon a successful bite attack, Clementine may grapple as a bonus action, with advantage if the creature is smaller than her, this is done with advantage.

Sit. Sits down. Must succeed a dc8 Intelligence roll plus the commanders animal handling modifier

Lay Down. Lays down. Must succeed a dc8 Intelligence roll plus the commanders animal handling modifier

Roll over. Rolls over. Must succeed a dc8 Intelligence roll plus the commanders animal handling modifier

Paw. Gives you her paw. Must succeed a dc8 Intelligence roll plus the commanders animal handling modifier

Speak. General mouth noises. Must succeed a dc8 Intelligence roll plus the commanders animal handling modifier

Sneeze. ACHOO. Must succeed a dc8 Intelligence roll plus the commanders animal handling modifier

Play Dead. acts as if dead. Must succeed a dc8 Intelligence roll plus the commanders animal handling modifier and a contested deception check

Fetch. fetch an item within 20 feet. Must succeed a dc8 Intelligence roll plus the commanders animal handling modifier

Catch. if an ally within 5 feet is disarmed, Clementine may use her reaction to catch said object. Picking the object back up would now be a bonus action apposed to an action.