The Devil's Demonic Pets
More than a pet, Cerberos could easily pass for a living weapon. Though his small size, he's extremely vicious and ready to bite everthing or everyone that comes across him, thinking of them as food. Hiruma think of him as a weapon to use to scare and motivate his team-mates during football training and apparently (according to one of the question/answers pages in one of the volumes) Cerberos himself doesn't stick around the blond devil because he's affectionated to him, but because he knows that by stay around him he can easily get food.
Also, according to an omake, at the first Cerberos was in apprearance a sweet and fluffy stray, but Hiruma's influence soon triggered his savage inner side and changed him into the wild fangy Hell Watchdog which he's at present.
I'm not quite sure if these can be considered pets at all, they just "pop up" from somewhere whenever Hiruma's doing something particularly devious.
Then again, they're most likely just Murata's ways of showing Hiruma's "aura of evilness", but let me ramble a bit anyway...
...my only guess about them is that they're attracted to him in the same way Kyou and Yuki from Fruits Basket attract respectively cats and mice when they're experiencing any kind of strong feeling (and of course I'm kidding here ;)).