Cockroach

Menial. Living Creatures are immune to Chomp attacks made by this creature, Undead are vulnerable to Chomp attacks from this creature.

Mutant. What doesn't kill it makes it stronger, When this creature escapes from combat it gains a permanent boost to its max health and damage equal to the damage it took in that encounter.

Horrid Creature. The cockroach rolls an intimidation check, all creatures within 10 ft. of it must make a wisdom save with a dc equal to the intimidation check or become frightened of the cockroach until the end of their next turn.

Chomp. Melee Weapon Attack: +7 to hit, reach 5 ft., 1 target. Hit: 1 [piercing] damage.

Scurry Away!!!. The Cockroach gains additional movement speed equal to a d20 times 5 for the rest of combat.

Why. Won't. You. Die!!!. When the Cockroach takes damage it can uses its reaction to reduce the damage by its walking speed.