Keen Smell. The rat has advantage on Wisdom (Perception) checks that rely on smell.

Undead Fortitude. If damage reduces the zombie 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 point instead.

Bite. Melee Weapon Attack: +0 to hit, reach 5 ft., one target. Hit: 1 piercing damage.