The Ghoul: deep cave monster

The ghoul would spawn below y level 0 and (maybe) in pale gardens in the overworld and in soul sand valleys in the nether. Ghouls would replace a portion of zombie spawns, perhaps scaling with depth.

The ghoul would add a little extra challenge and spice to deep caves. Their speed, high detection range, and sense of smell would make them prone to jumping players who thought they were alone.

Ghouls would be human shaped and player sized but skinny. It would be dark colored to blend in with deepslate and soulsand but not truly camoflauged. When aggroed, a very long tounge would hang out of its mouth and it would reach toward its target and bend forward a bit as it ran, the picture of mindless hunger. 

Ghouls would have a detection range of 35 blocks by sight and 12 blocks by smell. When it detects an enemy (player, villager, iron/snow golem) it would rush quickly to the target and attack relentlessly until one of them died. It would have 16 hp and deal (easy) 3, (medium) 4, (hard) 6 points of damage. Ghouls would be slightly faster than a running (not sprinting) player.

Ghouls would drop rotten flesh with a chance for mushrooms if killed by player/tame wolf. They would not be able to pick up any items and would drop items if forced to equip them.