Before Gen 6, you did have to see the Pokemon in game before you could search for it in the GTS. But, in Gen 6, if you scroll to the bottom of the list of letters when you enter the Pokemon you look for, there should be a button that says something like "Enter name." If you press that, you can enter the name of any Pokemon, it doesn't matter if you've seen it or not. I'm not sure if you have to capitalize the Pokemon's name, but I know that you have to spell it right.