Chomper

Tunneler. The Chomper can burrow through solid rock at half its burrow speed, and leaves a 1-foot-radius tunnel in its wake.

Undead Vanquisher. The Chomper has advantage on attack rolls against zombies. 

Chomp. Melee Weapon Attack: +5 to hit, reach 5 ft., 1 target. Hit: (2d10+3) piercing damage. 

Swallow. While burrowing, the Chomper can emerge under a creature in an attempt to eat them. If the target's size is Medium or smaller, they must succeed on a Dex 15 saving throw, or get swallowed by the Chomper. An undead creature has disadvantage on this saving throw unless it is Large or larger, in which case it does not make a saving throw. While swallowed, a creature is blinded and restrained, has total coverage from attacks outside the Chomper, and takes (3d6) acid damage at the start of each of the Chomper's turns. If the target falls to 0 hit points while inside the Chomper, they are digested. The Chomper can only swallow one creature at a time, and whilst it has a creature swallowed, its walking speed is reduced to 5 feet and it has disadvantage on Dex saving throws. If the Chomper falls to 0 hit points, a creature it has swallowed can crawl out of the corpse using 5 feet of movement. 

Goop. The Chomper spits purple goo at a target within 60 feet. Make a ranged weapon attack against the target. On a hit, the target is covered in the sticky goo. The target's walking speed is reduced by 20 feet and it has disadvantage on its Dex saving throws. Additionally, the target takes (1d4) acid damage at the end of each of its turns. These effects last until the end of the target's second turn after being hit. 

Hungry Ambush. On an attack of opportunity, the Chomper can attempt to swallow a creature that is facing away from it.