All sort of fantasy creatures like:
elves, orcs, fairies, griffins, goblins.
Also different mythical creatures:
vampires, sirens, cyclopses, harpies, hydras, merfolk,
And purely original creatures - like the mega construction workers - that expand the dungeon following the original orders given by the original crew thousands of years ago. Actually I want for each and every one of the creatures to have such a "purpose" that was given when they were human.
I was thinking that - we can have the elves to have been scholars or scientists. And they are going to be one or two of them that might still have some semblance of a human consciousness.
I still haven't taught of how these creatures fit in with the dungeon as a whole.