Swamp Ghoul is a zombie-type mob that gains health when damaging the player or a mob.
Ghouls would spawn in Swamps and maybe Roofed Forests. They would look similar to zombies and drowned but they would be grey, have tattered clothes, and have warts all over them. They'd also have a terrible posture and have a hunched back.
Behavior: When idle, they walk slowly around and make quiet groans. When they spot the player they make louder groans and start moving fast towards the player. They would have an internal short-duration sprint-bar for that.
They are capable of "stealing" the players health when damaging them.
Balance: They would either have less health or deal less damage than normal zombies due to their speed & lifesteal abilities. They are also unable to spawn with a chestplate because of their hunched back.