"Following the rise of Ishgard, hoarhounds all but disappeared from the Coerthas highlands. However, the shift in climate caused by the Calamity has seen the legendary beasts descend from their mountain dens once more."
this last bit here is so vague that it could justify fenrir mount flying lore wise anyway for example Dragons descend from homes and can fly, humanoids can descend from a mountain anyway they wish be it skis/falling/walking/snowboarding etc and can't fly the word 'descend' is left up to interpretation which i believe the devs are doing.
As for whether it should fly is down to the player base and the dev team response to the player base same with any other mount. i personally don't care if it can fly or not or any other mount for that matter, i don't want the devs however to spend too much time on making every mount fly when there are plenty of other things they can work on to improve. Its nice but not necessary within the game at this point in time
