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 attack rolls against a creature if at least one of the dog's allies is within 5 feet of the creature and the ally isn't incapacitated.

Undead Fortitude. If damage reduces the dog to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the dog drops to 1 hit point instead.

Formation Tactics. The dog has advantage on saving throws against being charmed, frightened, grappled, or restrained while it is within 5 feet of at least one ally.

Fierce Protector. During an owner’s long rest, any enemy or potential intruder within 20ft. must make (or repeat) a Dexterity (Stealth) check against the dog's Passive Perception. On a fail, the dog wakes up and uses its bark to alert all allies within 120ft. This takes the dog one action. 

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 7 (2d4 +2) piercing damage plus (1d4) necrotic damage. If the target is a creature, it must succeed on a DC 11 Strength saving throw or be grappled. While the creature is grappled, any further bite attacks automatically hit.

Elusive. The dog can take the dash or disengage action as a bonus action.