Patagium: a membrane or fold of skin between the forelimbs and hindlimbs on each side of a bat or gliding mammal.
This one was tough to train, and it isn't perfect. Some subjects are more inclined to produce feathered wings. In that case, prompt "patagium wings" or "membranous wings".
Often enough the output is not a true patagium wing. In other words, they do not stretch from the forelimbs to the sides of the creature. After all, a cow is a cow, with four legs. Still, with some persistence, you can produce an image that removes the forelimbs. Sometimes it just looks weird to do so.