Coincidence?
Here's a thing: Howard the Duck (1 August 1986) came out exactly 28 years to the day before Guardians of the Galaxy (1 August 2014). Coincidence?
DVDs I own: http://brandeks.filmaf.com/owned
Might not be. The director apparently said Howard's cameo was gag. So it's possible they had this planned for a while. I do hope the character comes back, though.
http://www.freewebs.com/demonictoys/