I've seen more than one fish called flying fox (they're similar looking, especially when small). At least two of them are pretty dicey for discus ime. One is related to rainbow sharks and red tailed black sharks and, like them, they can be peaceful when small but get more aggressive with size ime. If it's a siamese algae eater (sometimes mistaken for flying fox), these also tend to be peaceful when small but get more aggressive with size ime, and I've had them try to eat the slime coat of discus. I've seen someone occasionally say they had flying foxes (or Siamese algae eaters) with discus without a problem-- imo either they probably were still small at the time or maybe they just got the odd one that happened to work. I mean, I once saw a green terror kept with discus and they seemed to get along, but I wouldn't exactly recommend it. But most people I know of who've tried flying foxes and discus say more or less the same as what I've experienced.
There's another one, sometimes called false flying fox or stone lapping minnow. Don't know about behavioral compatibility with these and discus, but they like cool water according to profiles I've read.
