Cerberus is a massive three-headed dog, charged with guarding the gates of the Underworld to prevent the dead from escaping. He is a household pet and companion for Hades and Zagreus, and he is a very very very good boy.
When Zagreus left for the first time, Cerberus threw a tantrum that wrecked a large portion of the House's lounge area.
||'...The fearsome multi-headed hound of hell is really not so bad as his deserved reputation otherwise would lead you to believe. Yes, the massive beast has feasted on no short supply of interlopers to the Underworld, or on those attempting to escape. However, tales of the number of his heads are oft exaggerated, and in his old age, his temperament has grown much more relaxed. Indeed, he seems one of the sole sources of comfort for the House's grim Master. The fact that Cerberus now frequently rests at his Master's side also suggests the Underworld has new vulnerabilities, a source of some concern to the Lord Hades, who understandably takes pride in his domain's imposing reputation.'
For additional information on Cerberus that does not pertain to Hades, see Wikipedia's article: Cerberus