I noticed a few people asking for things like Mew, Reshiram, Rayquaza, etc. Last I knew, nobody has hacked into the games yet and there's no legitimate way to see those Pokemon either. You also can't ask for Pokemon you haven't at least seen on the GTS, right? So how are people asking for these?
Shadow Temple... Here is gathered Hyrule's bloody history of greed and hatred... - Shadow Temple Walls
You can search and ask for any Pokemon that is programmed into the game as long as you know how to spell its name. In the GTS' alphabetical list of Pokemon, just below the letter Z, is an option "What Pokemon?" click that and you can type in the name of any Pokemon to search or ask for, even if you haven't seen it.
3DS FC: 4012 3979 5838 Now Playing: Pokemon Y, SMT IV