Exiled Future has nearly every monster you can find in any D&D monster manual or any RPG monster book. Yet there are a few monsters that are known across much of the underworld that adventurers keep an eye out for.
Goblin Tree – A predatory tree that drops huge fruits that become prowling creatures that kill and drag their prey back to feed the tree. One or two trees is a minor nuisance. A forest of them is something to be avoided.
Drow – Elves that hate all lesser races and have banded together to plot and plan the destruction of all other sapient life to remove the source of their pain. The main distinction between Drow and their more friendly kin is the bat ears they have adopted to better live underground in the dark.
The primary livestock and beasts of burden are giant lizards. Meat in towns is pretty much always giant lizard, standard leather is from giant lizards, and giant lizards are what you will see pulling wagons. Not being ridden though, they make terrible mounts.
Others to come as I think of them. Not to be considered anything close to a full bestiary of monsters you might find in the underworld.
Another detail to keep in mind that the wild nature of magic and the general broken nature of reality in some parts of the underworld means that many places have monsters that are just mutated natural animals of one sort or another. Adventurers that survive get quite skilled at spotting the signs of such warped animals.