So this will be the animated crossover episode, where Sam and Dean ended up meeting Scooby Doo (and the gang, far as I know?).

From Ausiello (about this ep.):
Ausiello: At Comic-Con, Jensen Ackles teased that the installment is “very Scooby-Doo-esque,” but “slightly darker.” And, yes, the boys will definitely get animated. (Ackles’ reaction to his cartoon self? “‘Yes!’ How cool is that? I grew up watching Scooby-Doo. Now I’m, like, on it.”)