Guardians of the Galaxy director James Gunn confirmed the news to Empire Online in a "spoiler" podcast where Gunn gave a rundown of the items featured in the collection.
Note: bold my emphasis:
"Many people may look up to the right of The Collector. As we first see The Collector, we push in on him and you will see a certain duck named Howard that turns to look at the group. That's one of my favorite little things. The background there is the Slither creatures from my movie Slither behind The Collector; those guys are pretty obvious. You have Adam Warlock's cocoon. You have also sorts of characters from other Marvel movies. The Dark Elf back there, who is played by Doug Jones, who is one of our stunt guys that had the Dark Elf make-up on. So you had a lot of different guys in the background."
Gunn also adds that The Collector has more than one "museum," with the one shown in Guardians of the Galaxy being the Knowhere "wing."