Ny Lumiosity City map

I had the same trouble finding this option at first, and argued with someone at length that this just wasn't possible. The letters list does seem to be like the old GTS; you can only search for Pokémon you've at least seen.

But if you scroll all the way down past "Z" You'll notice a button called "What Pokemon?" Click on this and it will bring up a text input, where you can manually type in the name of a as-yet-unseen Pokémon and search for it.

It is not possible to shorten a Pokémon's name to search for it. Searching for "Geodude" will return results, but not "Geo". However, do be aware that it does not inform you whether or not the name you typed exists - only that there were (no) results after you search. Also as there are a number of Pokémon that do not normally occur in the Kalos region, searching for them will likely not produce any result until Nintendo releases the Pokémon Bank and this generation's transfer tool.

Also, as event-only Pokémon cannot be traded on the GTS, there are none listed. So if you've manually searched for "Mew" or "Celebi" or any of the other event-distribution Pokémon, it will not yield any results, as there ARE none of those Pokémon for trade on the GTS.

I'm guessing it has something to do with either Rotom or Cresselia in X/Y (I'm playing X and I got the eerie-hex-maniac-girl-ghost too). The Strange House (at the base of Reversal Mountain) in Black2/White2 is where you get the Lunar Wing to catch Cresselia on Marvelous Bridge (also haunted), and in Diamond/Pearl, Rotom can be found in the haunted Old Chateau in Eterna Forest. I could be wrong of course, it's just a theory. There's still LOADS of stuff we don't know about X/Y yet. It might even have something to do with Darkrai, for all I know. Sorry I couldn't be of much help.