I have this idea of giant, literal flying fish. About the size of a megalodon (if impossible, it can be smaller, I just want them to be really big), they fly at speeds of 70 miles per hour (again, you can change the speed if that's too slow/fast), and they, well, have to be able to swim and fly. Could they evole, and if so, how would they fly?
The gravity isn't as strong as it is on Earth, and the atmosphere is thicker. Just how weak the gravity and thick the atmosphere, I haven't decided, though.