Imagine a tribe of nomads where all the males have the blessing of being were-stallions. The tribe would not need to have ordinary horses to move around, all mounted warriors would be female and a curious custom could be that when a couple gets married, the girl rides her chosen to the altar.
Near a major city, a mirror image of the place has appeared. This strange double is infested with hostile insectoid humanoids that appear to match the city folk in numbers and armaments. The humans’ diviners have determined that the secret of the city’s appearance can be found within the double’s college of magic, so heroes are needed to explore within this mirror city and discover what has drawn it from its distant realm. There isn’t much time to spare, as the bug folk find their mammalian counterparts unnatural and blasphemous, and plan to destroy them.