Undead Fortitude. If damage reduces the Zugg 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.

Mark of the Plague. If damage was done with the bite action to a creature then they must succeed a Constitution saving throw of the damage taken or suffer the Zugg disease.

Horde Mentality. Zugg tend to stay and travel in great numbers across the land and even in battle. If one is alone they will seek out a herd to join with.

Movement is key. If there are no humanoid creatures within visible range of the Zugg then they will travel in a random direction until provoked to another one. The goal of this is to find food.

Steal from Thy Dead. When a creature is transformed into a Zugg they will retain small memories and habits of the past life. They may take an action from the past stat block and use it for itself. Concentration spells cannot work when a creature that it sees as food is around. 

Slam. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 4 (1d6 + 1) bludgeoning damage.

Bite. Melee Weapon Attack: +3 to hit, reach 5 ft, one target. Hit: 1d8+2 piercing damage and make a Constitution saving throw of the damage taken