Keldeo looks... horrible (sprite) yet godly (stats, typing, etc).
If it runs HP Ghost, you've got Dragon/Flying, Gyarados, and Grass/Poison standing a decent shot. Dragonite (Hello Hurricane variants), Gyarados, Amoongus, Venusaur, and Roserade probably being the best answers. Though none are particularly suited to Sand teams.
If it runs HP Ice (trolol no Ice Beam, welcome to the Seismitoad club), you have the Water/Psychics, Gyarados again, Jellicent, and Shedinja (it could almost work. Almost) and Tentacruel again.
The coverage won't be perfect, even though Fighting+Ghost is obviously unresisted an un-STABed HP Ghost doesn't seem... scary. Gyarados and Tentacruel wall it either way, but seriously if you don't have Gyarados or Tentacruel good luck. Or maybe Jellicent's Troll Body will disable HP Ghost. Other than that, I can't see a very big list of things to counter it. But who knows, a Rain+STAB+LO Hydro Pump coming from 129 Base Sp. Atk could maybe 2HKO, I don't have damage calcs on hand.
So yeah... I think it'll be a bad day for OU, but I think it'll go Uber.
Either way, I'll have a Magic Guard+Sash Alakazam waiting. Because that freak horse is going down. Even though it'd require death fodder to get in.
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