Bora Dog

Preferred Person. When introduced into a new pack, the dog will study the group for up to twenty four hours. After this time, it will choose someone to become their preferred person. They will follow this person around and becomes fond of them, rarely leaving their side and displaying acts of extreme loyalty. They will never go back on their choice while in the pack. They can obey simple commands. They perform special abilities in combat.

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

Pack Tactics. The dog has advantage on an attack roll against a creature if at least one of the hound's allies is within 5 feet of the creature and the ally isn't incapacitated.

Beastly Transformation. Once every long rest, provided that the dog is within 100 ft of its favored companion, it can transform into a large sized version of itself. Transformation lasts 1 minute. While in this form:

AC: +2

Speed: +10

Bite Attack: +2 to hit

Green Flame: Exhales a 30 ft cone instead of 15 ft. DC 14 Dexterity save. On a fail, targets take 10D6 fire damage. Half as much on a success. 

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) piercing damage plus 7 (2d6) fire damage.

Green Fire (Recharge 5–6). The dog exhales fire in a 15-foot cone. Each creature in that area must make a DC 12 Dexterity saving throw, taking 21 (6d6) fire damage on a failed save, or half as much damage on a successful one.

Worried Whimper. When a teammate has taken more than 10 damage directly before the dog takes its turn, the dog will let out a small worried cry. +1 HP healed to target. If directed at its preferred person, +5. If a target is brought to zero HP, the whimper automatically gives the character one succeeded death save. 

Heel Hound. Within five feet of a companion, the dog will perform a bite attack on anything that attacks. This attack has automatic disadvantage. Once per round.