I haven't seen this posted yet, and didn't find anything like this when I did a search and I dunno if this is a exploit or neat little trick, but this is it. Also, I dunno if this will work for anyone else, but it has worked for all 3 of my flying mounts, and i'm pretty sure its client side only. Anyways, Hope you enjoy.
To make your mount fly upside down/ do a loop you must:
1) Have some sort of flying mount ( i tested with a drake, epic bird, and normal bird)
2) Have your Num Lock key turned off ( the numlock light shouldn't be on)
3) And while flying, hold down your your Numpad 0 key, and your mount should do a loop/start flying upside down if you let go midway through.
Here are some screenshots of me doing this.
I'm sorry if this doesn't work for anyone else, or if it was already posted, or if you find it a waste of time, though i thought it was kinda neat.