Flying mounts wouldn't be practical for a number of technical reasons, including load times, worldspaces, accessing areas you shouldn't and game balance.
Agree with posters 3, 5 and 6. Unnecessary and ridiculous! This isn't some MMO, the physical size of Skyrim isn't massive compared to those games so flying mounts, of any kind are out of context and would ruin/break gameplay in so many ways it'd make your head spin!