Now in order to feed all those underworld nasties,
I propose the UTOW
(feel free to create your own stats):
Utows are a ubiquitous worm that have the unique ability to extract heat energy from the planet’s soil and convert it to biologic mass. The planet has a molten core, that radiates heat at an unsteady and alarming rate. Utows, aka ubi-thermo-worms play an important role in maintaining environmental stability by cooling the surface; otherwise, internally-generated heat from the core would burn and desiccate the root systems of the planet's flora.
Although not usually large (size = 2d6 -3), nor tasty; the utows are nutritious and are the foundation of the diet of subterranean reptiles, amphibians and humanoids. Utows are a subterranean krill that have their greatest concentration 10 to 20 feet below the surface. In general, larger utows are found deeper below the surface. Naturally, utows are rich in vitamin A, a key component of retinol (infravision).
Neither agile or aggressive, utows can deliver a savage bite and the worm’s gizzard will contain (1d6 x Size) coins of varying denominations.
the Dungeon Dozen
has some more creative ideas