Ah, the Marvel post-credits scene. Ever since Samuel L. Jackson showed up at the end of Iron Man with an eye patch and some foreboding words about the "Avengers Initiative" they have taken on a life of their own.
They've teased sequels, sequels to sequels and even explored the wonder that is shawarma. There have been multiple post-credits scenes for a movie, there have been some that require explanation from comics fans, but there has never been one quite like this.
(Serious Guardians of the Galaxy spoilers follow!)
In the post-credits scene in Guardians of the Galaxy we see Benicio Del Toro's Collector moping with a drink next to his destroyed Collection. A dog in a Soviet space suit comes over and licks his face. Then we hear a voice ask, "What do you let him lick you like that for?" and the camera pans over to Howard. The. Freaking. Duck. Howard says, "Gross," and then the movie is officially over.
If you were confused, don't worry. We're here to help.
Howard the Duck is actually a Marvel Comics character created in the 1970s from the alternate universe of Duckworld (just go with it). He's kind of a grump, likes cigars and Earth ladies and actually already had his own 1986 movie, where he was portrayed by an animatronic puppet a lot like the ones you'd see at Disney World. It's pretty much considered to be one of the worst movies of all time, and for good reason. It's also kind of terrifying.
There's no way to tell just from that scene if Howard is going to figure into the Marvel Cinematic Universe again. The studio did just pull off a movie with a raccoon and a tree as main characters, so we wouldn't put it past it. It also might just be one more comedic random act from a movie chock full of the wonderfully weird. It is also definitely the first end credits sequence to totally surprise us, which gives us hope for the future ones. If Howard the Duck can pop up, who knows what else could happen?