For example, you could maybe ride a whale or sth like that in an ocean.

Personally, i think i would like to have blue whale lol.
BUT ANYWAY, would it be great to include the possibility to fly with your mounts?
As of this concern, i think the dervs in guildwars would tight this to the storyline, cuz what's the point of flying without telling why in the first place to do so??
Anyway, back to the question: If you could mount in GW2, what monsters or creatures would you want it to be? And what kind of terrains do you think it should be able to walk on?