Mages can make just about any creature into another creature through magic. This can be voluntary or involuntary. This can also be done to humanoids.
Create creatures from the stuff of flux. Some will dissolve when forgotten, but others will remain until killed. This could be a source of new monsters. Mages make new monsters based upon their powers and how much energy they want to tie up in each creature. Some creatures live off of the energy in the flux, while others have to eat real food.