It was a delightful surprise when Marvel's the Collector (AKA Taneleer Tivan) popped up in the post-credits scene of Thor: The Dark World. That was compounded when the MCU's next cosmic adventure, Guardians of the Galaxy, featured not only Tivan, but his famed museum.
Taneleer Tivan (Benicio del Toro) is one of the Elders of the Universe - a sibling of fellow Elder the Grandmaster, played by Jeff Goldblum in Thor: Ragnarok - and integral to the history of the MCU. Being immortal, the Collector fills his time acquiring all known things in the universe, living or otherwise.
Tivan's menagerie put a lot of immediately recognizable things on display in Guardians of the Galaxy (like Cosmo and Howard the Duck), but there were plenty of other, less easily identifiable objects and creatures, too.
Cosmo The Spacedog
Cosmo the Spacedog has a richer backstory than expected. The subject of Soviet experimentation, Cosmo was sent to space and ended up on Knowhere (the Collector's base of operations in the MCU). He absorbed cosmic rays along the way, which gave him telepathic powers.
Upon arrival, Cosmo teamed up with Nova to solve some murder mysteries, and ultimately became Knowhere's chief of security. He even become a member of the Guardians in the books. He seems to lack his abilities and job in the movie, but Cosmos's inclusion is quite fitting.
Howard The Duck
Howard the Duck was created way back in the '70s as a satirical, irreverent character with a metafictional awareness of his own comic book environment. He was basically the proto-Deadpool. The character even garnered a feature film in 1986, despite only possessing a cult following.
It's difficult to say if the animatronic version of the character in his '80s adventure is more or less creepy than the CGI version in GotG, but fans were certainly delighted to see him.
A Sovereign Cocoon
This cocoon has stirred up a bit of controversy, or perhaps more accurately, confusion. When comic book fans saw the cocoon in the post-credits scene of Thor: The Dark World, and then again in Guardians of the Galaxy, they assumed it belonged to Adam Warlock. At the time, director James Gunn assured fans it wasn't Warlock, just a fun thing he found in the comics and wanted to include.
But since Adam Warlock's actual cocoon shows up in the second GotG, Gunn has done a bit of retconning. He tweeting, "The cocoon in the 1st GotG (& at Disneyland) is likely a primitive version of the Sovereign cocoons, when they 1st hatched their own." This adds a few interesting layers to Warlock's new MCU origin.
Since the Sovereign were a race created specifically for the film, Warlock's affiliation was already a new wrinkle. Since we know the Sovereign themselves are birthed from these cocoons, however, we can infer that Warlock isn't just a creation of the Sovereign, but an actual member of their race.
A Dark Elf
Dark Elves, the villains of Thor: The Dark World, hail from Svartalfheim and are one of the universes oldest species. Though not explicitly stated, one might infer the Collector's acquisition of this particular Dark Elf seen in GotG might have something to do with said Thor sequel. A couple of Asgardians brought an Infinity Stone to the Collector in The Dark World's post-credits scene, and may have also given him a Dark Elf.