War Dog

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

Bonding. A humanoid creature can make a DC 12 Animal Handling check to become bonded to the War Dog (this roll is made with disadvantage if the creature has previously been in combat against the dog). On a success, the dog bonds to the creature and will protect it with its life, and obey its commands. On a failure, bonding cannot be attempted until the next day. 

Pack Tactics. The wolf has advantage on attack rolls against a creature if the dogs bonded owner is within 10 feet of the dog.

Reckless Rage. If the creature the dog is bonded with is knocked unconscious and is within line of sight of the dog, it flies into a wild rage. While in a reckless rage state, all attacks the dog makes are at advantage and deal an additional d6 damage. In addition, all attacks made against the dog are at advantage while it is in a rage.

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

Claw. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 1d4 + 1 piercing damage.