Arcanine

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

Lethal Fury. When Arcanine drops to half its hit points or lower, its attacks become more ferocious. On its turn, it can make one bite attack as a bonus action. This effect lasts until the end of combat.

Multiattack.  Arcanine makes two bite attacks. 

Mountaineer. A Medium humanoid creature proficient in Athletics can ride Arcanine as a mount. While mounted, the rider can use Arcanine's speed. Additionally, the rider has advantage on Dexterity saving throws made to avoid falling off, and the rider isn't incapacitated by being knocked prone while on Arcanine. Arcanine can use its reaction to push back a creature Grappling the rider (DC Strength contest).

Bite. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 7 (1d10 + 6)

Fiery Breath  (Twice Per Long Rest). The Arcanine exhales fire in a 30-foot cone. Each creature in that area must make a DC 18 Dexterity saving throw, taking 19 (4d6+7) fire damage on a failed save, or half as much damage on a successful one.

Intimidating Roar (Recharge 5-6). Arcanine lets out a fearsome roar. Each creature within 10 feet of it must succeed on a DC 17 Wisdom saving throw or be frightened until the end of its next turn. A creature can repeat the saving throw at the end of its turn, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the Arcanine Intimidating Roar for the next 1 hour.