Cosmo is a Golden Retriever/Labrador cross-breed dog that was a former test animal of the Soviet Space Program. He was launched into Earth's orbit as part of an experiment at some point during the 1960's, but ended up drifting off into deep space. Somewhere along the line, Cosmo was mutated and developed telepathic abilities, and he wound up arriving in Knowhere. He eventually took the position as head of security of the space station.
Cosmo has since become the liaison to the new Guardians of the Galaxy and assists them in plotting their passport destination and travel to trouble spots in the universe via the Continuum Cortex.
Cosmo was created by writers Andy Lanning and Dan Abnett, and artist Wellinton Alves. He first appeared in 2008's Nova #8.
Major Story Arcs
Cosmo's loyalty to the team was in doubt after a group of Skrulls infiltrated Knowhere and destroyed the Continuum Cortex. Cosmo was protecting a group of Skrulls and was accused as a traitor by Adam Warlock. Cosmo convinced Adam Warlock to see the truth as he did with Drax and Quasar.
Unfortunately things became complicated when Cynosure led the Luminals into Knowhere to kill the Skrulls, forcing Cosmo to protect the same group of Skrulls. Cosmo would have been killed by Cynosure's energy blast if it wasn't for a Skrull who threw himself to take the hit to save Cosmo. Cosmo was enraged and unleashed a telekinetic blast that stunned both the Luminals and Guardians of the Galaxy. Cosmo used this opportunity to explain and apologize for any confusion he had caused.
War of Kings
After some negotiating failures of the Guardians while trying to convince the Inhumans and the Imperial Guard to stop their War of Kings, Cosmo is one of the Guardians taken by Starhawk to the future. They learn that it is indeed Black Bolt that causes the Fault and the destruction of the universe.
With their time machine broken they formulate a plan to send a message back to the past to warn the other Guardians. Cosmo is able to telepathically communicate with one of the Celestials in the Celestial Engine (a device made by the Badoon to stop the spread of the Fault). The message is given and received by Adam Warlock in the past through Knowhere, which is able to communicate with other Celestials in any time period.
When the alignment of the Celestials in the engine breaks, the Fault consumes them. This should have been the death of Cosmo and the others, but it was not. They ended up being sent adrift in the time stream. As a side effect of this, Cosmo was made much younger into a puppy.
Only after traveling through several alternate futures are Cosmo and the other Guardians assisted by Kang the Conqueror, who restores them to their normal states. He informs them of Adam Warlock becoming the Magus and that all futures are becoming Magus futures. He gives Starlord a Cosmic Cube to stop this from happening and transports them to moments before the transformation.
Starlord is unable to stop Adam from becoming the Magus. When Cosmo and Mantis try to combine their telepathic abilities to assault the Magus, he returns with an attack of his own, killing them both. Starlord is able to stop the Magus, but not before Cosmo, Mantis, Major Victory, Phyla, Gamora and Adam have lost their lives. This is what the Magus led everyone to believe and Cosmo and the others were taken prisoner and tortured in an attempt to convert them to the Church's belief. They were able to escape but soon came face to face with the evil Titan Thanos.
The true nature of the Fault began to reveal itself and the corrupted beings of the Cancerverse began their invasion. The Guardians allied with Thanos to stop the evil Lord Mar-Vell. In the end, the team disbanded, losing Warlock, Phyla, Drax and their leader Starlord. But before he died, Starlord left Cosmo with a task - collect the greatest heroes in the universe to form a band of " Annihilators," the team Peter believed was what the Guardians of the Galaxy should have been.
Cosmo one by one convinced the Silver Surfer, Gladiator, Beta Ray Bill, Quasar and Ronan to team up and protect the universe together, as was Starlord's final wish.
Powers & Abilities
In addition to a dog's standard enhanced sense of hearing and smell, Cosmo has psionic abilities, including high-level telepathy and telekinesis. He has shown to be capable of creating defensive shields strong enough to deflect energy blasts, as well as project mind blasts of tremendous force. His exact power levels are unknown, but he has shown to be strong enough to take on a being such as Adam Warlock by himself, and powerful enough to unleash a telekinetic blast able to disable two separate teams of superhumans.
In addition to these abilities, Cosmo seems to have a considerably extended lifespan as well - especially for a dog since he has been around since the time of the Soviet Union.
In other media
Guardians of the Galaxy (Film)
Cosmo appear in a post-end credits scene of the film as one of exhibit in the Collector's museam,he's freed after a huge explosion cause by an Infinity Stone destroy his cell.