Kerberos ("Kirby" for short) was the name Amadeus Cho eventually gave to a coyote cub he'd adopted after his mother was hit by a car. Kirby was Cho's companion for some time.
In the aftermath of World War Hulk, Amadeus was jumped by Skrulls and knocked out. Kirby was then replaced by a Skrull.
During the Secret Invasion, Amadeus and the Skrull Kirby joined the God Squad in their mission to destroy the Skrull Pantheon. On the way there, the Skrull Kirby tried to sabotage them, but had his head bitten off by Demogorge, revealing his deception.
Afterwards, Amadeus searched for the true Kirby. With help from the Hulk, he found him, but discovered that Kirby had grown and found a mate. Amadeus decided to leave Kirby there, realising his life was too dangerous for him.