I know I may be late to the party, but I want to throw my hat into the ring for the challenge posted by one Judd Karlman.
City : Surhan
The city forced to move itself underground when a meteor was predicted by the prophets of hitting continent and send the world into a Great Darkness. The giant maze of caverns used to save as many people as they could.
Formian - These creatures were instrumental in helping move the city and it's occupants underground. The city's leaders kidnapped the hive queen in order to force their cooperation. She continues to lay eggs which are then enchanted to follow the orders of the Hive Caretakers. They are basically mindless slaves and do as the city needs them to do.
Succubi/Incubi - A small pack of them have made a deal with the leaders of the city to bring pleasure where there is mostly pain. They use their powers to help the masses forget that they live under a huge mountain. Every person of Surhan gets a token monthly to visit the pleasure caves. Young or old. Some question the sanity of this plan dealing with demons, but so far, no one has died.
Minotaur - These volunteered to be guides for the many citizens of Surhan, without which they would be lost. No one knows why they showed up, but the city wouldn't run without them.
Delvers - These creatures are the main city builders. Unfortunately due to their various tastes for different rocks, the city is not built in straight lines. They are kept under control by special talismans that the Builder's Guild wear.
Air Elementals - Bounded to various caverns to help supply oxygen as needed to the masses. They are also used to control the masses, if a riot breaks out, the Mages of the Unseen Hand can and will lower the oxygen level in order to keep peace. The citizens know this for what it is and their is a quiet rebellion brewing.
Hydras - City's food supply. They managed to capture four of these creatures to use their regenerative powers for an endless supply of food. On a child's sixteenth anniversary of their birth, they are given a hydra head as their birthday meal. The brain is considered a delicacy.
After writing this up, I noticed that there was a common theme with some of the other people's ideas. Oh well. Enjoy.