There are a bunch of others too, ex. Latias, Dialga/Palkia/Giratina, etc.
I just wanted a freaking level 60 or higher Latios for a Lvl. 59 Reshiram, what's wrong with that!?
Its probably because you havent seen them in the pokedex yet. You can't get a Pokemon from the gts until youve seen them.