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

Armor Proficiency.  The mastiff is proficient in all armor.

Attacker. The mastiff gains a +2 bonus to all attack rolls (which is accounted for in its Bite attack).

Improved Critical.  The mastiff's attack rolls score a critical hit on a roll of 19 or 20 on the d20.

Bite. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 5 (1d6 + 2) piercing damage. If the target is a creature, it must succeed on a DC 13 Strength saving throw or be knocked prone.

Second Wind.  The mastiff can use a bonus action on its turn to regain hit points equal to 1d10 + 5. Once it uses this feature, it must finish a rest before it can use it again.