So for my upcoming Pokemon Metro Project fangame this is going to be the main (and probably only) legendary Pokemon. Since the whole city region is in the sky I had to go with a flying type; plus having bird-like wings kinda ties in with the cage element of the game. The shark part of it's design refers to what it represents which is the "insatiable hunger"; not in the sense of battle but of eating.
This legendary known only as "Oblivion" can't be tamed; it will eat people, Pokemon and if it could see it's own reflection it would probably try to eat itself. So when the time comes that you have to fight it you won't be given the catching option. And you might be wondering, if the whole region is a collection of cities then where are you supposed to find this thing? Without spoiling anything, lets just say it's right under your nose. ^^
Oblivion (c) Me