Ponies and Undertale meet once again, this time in a fan created fight made by none other than the DoubleWBrothers called Pinkielovania! That's pretty much the only clue I can give you without saying any Undertale spoilers, so check on after the break if you want to be spoiled or if you've played through Undertale.
Thanks to Masquerade for sending it in!
Twitter: Calpain