It is the Event Darkrai and they are blocked from putting them up as well as trading them to people asking for them. I think the GTS should block people asking for Pokémon that are untradable on the GTS. Why does it allow those requests when the system won't let you do it?
Well......
In theory most event exclusives should be available without ribbons, therefore blocking people from requesting Mew and the like outright defeats the purpose of blocking the ribbons (which really also prevent the trading of events that had special moves, etc).
Mew and Deoxys both had events in Gen III where they could be caught in the wild (Mew was only in Japan).
Celebi and Jirachi were a preorder bonus for Colloseum (celebi being exvlusive to Japan)
Gen IV saw Darkrai and Shaymin both events that uou could catch in game. Manaphy was an egg event via the Ranger games.
Gen V saw Victini the same way, and saw Arceus as a Dreamworld Event.
So far only Keldeo, Meloetta, and Genesect are only available with a ribbon that would preventball trafing.
You really should have just shopped here for a 6IV Ditto. I got mine here. Now I breed like a boss. It took me about an hour to catch a 0IV mother and breed it up to a 5IV with the proper spread. If I start with a Safari mother it's even faster. Look here for all your trades for certain IV. The GTS is just for Shiny/Legendary hunting.
Yeah, well I was acctually not looking for a 6iv Ditto, I was looking for a shiny one, but when I saw the claim of 6iv I thought on it. The only reason I snagged it was the claim og "rng in b/w" which made me trust a bit too much.
As you said though, the GTS is good for shiny hunting and legendaries. I snagged a shiny Magikarp for onlyna Ditto, a Kyogre for a Tottodile, and a shiny Gyrados for a Chimchar. Not to mention the extras of the three birds, and X/Y mascots I was able to snag for only starters that I was able to recycle as trade fodder for other legendaries. At this point I am missing only 7 legendaries (all from gen 4 and gen 5)... other than the event exclusives.
As I bring up though, the event exclusives feel possible because of all the ticket events, the gen 3 events without ribbons, and the dream world Arceus.
I am not overly heartbroken over the ditto I pulled off the gts. I am unhappy that someone would so blatantly lie. I think that was scummy of them... but I have pulled so many trades that are much more in my favor, and had only the one lie.... so I have nothing in the long run to complain about. And a lesson learned.
Oh, my favorite is all the people who claim a Pokemon is shiny when the GTS luckily lets you see that status... and those that claim Pokerus when the pokemon has a cured strain and can not pass it on. I also find funny that people will ask "shiny only please" in their requests. There is no way to enforce that request, and I sm sure a lot ofbpeople get heartbroken over it.