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

Undead Fortitude. If damage reduces the hound 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 zombie drops to 1 hit points instead.

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 4 (1d4+2) piercing damage. If the target is not undead or a construct, they must succeed on a DC 11 Constitution saving throw or become infected. The infected creature is Poisoned for 1 hour.