Crowley's hellhound

Growley was a hellhound owned by the King of Hell Crowley.

History
Growley is a hellhound about five feet in height, reaching nearly its master's shoulder, that has been owned by Crowley since its debut and acts as a pet, bodyguard and assassin.

Season 5
In The Devil You Know, when demons send a hellhound after Brady and Crowley, Crowley decides to bring his own hellhound Growley to fend off the other hellhound while he, Sam, Dean and Brady make their escape. The two hellhounds battle in a vicious scuffle that presumably ends in the other hellhound's death and defeat. Crowley remarks that he is confident in Growley's winning the fight.

Season 6
In Weekend at Bobby's, when Bobby Singer traps Crowley inside a devil's trap in attempt to get the demon to break his contract, Growley is also present as Crowley's means to convince Bobby to let him go, or otherwise lose his life. Bobby reluctantly agrees to avoid being killed.

Season 8
In Trial and Error, Growley is sent to kill Carl Granville, Margot Cassity and Ellie to collect their souls as part of the deals they made ten years ago with Crowley. Before Sam and Dean can intervene, Growley manages to kill Carl and Margot and drag their souls to Hell, but struggles to get to Ellie due to Dean being able to see it using glasses burned with holy fire. Dean attempts to kill the hellhound to complete the first trial, but Growley manages to get the upper hand and nearly kills Dean.

Sam disrupts its attack leading Growley to attack Sam next, only to be killed by Sam when the hunter slices its belly open with the demon-killing knife. Its blood drenches Sam as per the trial's requirements. Due to its demise, Ellie is spared from her contract.

In Goodbye Stranger, Crowley mentions his desire to get revenge on Sam for killing his poor dog.

Weaknesses

 * Demon-Killing Knife - Sam killed Growley by slicing it open with this knife.

Trivia

 * Growley is a fan nickname. This particular hellhound is never named on-screen.
 * In season 9 episode "King of the Damned", Crowley has replaced Growley with a hellhound named Juliet. Unlike Growley, this hellhound is named on-screen.
 * Growley is the first hellhound to appear visible to the audience and Sam and Dean.