Spoiler Warning: This article contains a huge surprise spoiler for Guardians of the Galaxy, so only keep reading if you want to know. While the spoiler is likely not an integral part of the plot, it is a spoiler that you might prefer to be surprised by. You have been warned.
Disney and Marvel Studios released the full, credited cast list for Guardians of the Galaxy to Stitch Kingdom, and in doing so, they revealed a huge spoiler for the film. For the most part, the credit list included cameos that had already been confirmed. However, while many had been guessing that Nova might have a surprise cameo in the film, it's actually someone else from the Marvel universe that shows up.
In the end credits, there is a line that credits Steve Gerber and Val Mayerik, who are the co-creators on none other than Howard the Duck. Yes, Howard the Duck evidently has a cameo in Guardians of the Galaxy. How Howard the Duck will cameo in the film is unknown. It could be anything from Peter Quill reading a Howard the Duck comic book or watching the Howard the Duck movie to Howard the Duck appearing as one of the aliens in the film.
Whether this signals that Marvel Studios is planning a Howard the Duck reboot movie, we have no idea. More than likely, it's just a cool brief cameo of the character, probably included at the request of director James Gunn.
Guardians of the Galaxy is scheduled to be released in movie theaters on August 1, 2014.