Each of his heads has a distinct personality, so it's like having three dogs in one! Given his purported role as defender of the Underworld, it's unsurprising that he tries like hell to keep intruders away from your house... and may even attempt to keep you from going out!
Although he's depicted with three heads today, Cerberus was never that consistent in the myths: art usually showed him with two, the first source to describe him said 50, others said 100, one said he had a single dog head and 100 snake heads, Euripedes described him as having three bodies, which... wouldn't that just be three dogs, then? And although this is supposed to be a vicious guard dog, Hades (gigantic nerd that he is) gave him a name that basically translates as "Spot."
Since the Cryptkins are all about baby monsters, Cerberus is still a puppy, with round, short-nosed puppy heads, each with a different look: one with his mouth open wide, ready to play; the second with its tongue hanging all the way out and the right ear flopped over; and the third blepping. The body is a dark blue-grey, while the large round eyes are red. A brown studded collar stretches around all three necks, and to make him look less average, his tail has spines like a dragon.
Bagged separately inside the box with each figure is a small card with art of the Cryptkin on one side and biographical facts (well, "facts") on the other: name, full-grown height/weight, descriptions, diet, and habitat and range, with a map of the world to help drive that last one home.
Cerberus' art shows him (them?) standing on a fiery underworld cliffside, because Disney's 1934 short The Goddess of Spring conflated Hades (the god) with Satan, and thus Hades (the location) with Hell, despite the fact that neither of them were like that in the first place.
There are two Cerberus variants available. The Cryptozoic Entertainment online store had an exclusive "Damnation Cryptkin" that was basically a dalmatian version and was limited to 300 pieces that sold out before most people even knew it existed. Shame, because it was cute, with spots that looked like skulls and its own unique card, showing him sitting in a desolate wasteland. Oh well.
The other was available at Hot Topic, one of two exclusive Series 2 figures they got.
Folklore describes how dangerous it is to stare into his eyes (and he has so many of them!), but there is only one real danger... that you will fall in love! He's got a fiery personality - seriously, sometimes he shoots fire out of his mouths. But he's loyal as hell and has three times the charm of any dog you've ever met!
Hellhounds are generally just normal(ish) dogs, not ones with three heads. Still, this variant is very easily distinguished from Cerberus proper: though most hellhounds are described as being black with burning red eyes, that look was already taken by Cerberus, so this one instead is a warm, cinnamony red, rustier than the red of the eyes, and he's been painted with white bones on his surface, similar to the rare Bone-Chilling variants some of the other molds got.
The art card shows Hellhound standing in a church's graveyard with a bone in one of his mouths, and the "Habitat and Range" section of the bio on the back specifies this is meant to be somewhere in Great Britain - the concept of a hellhound is pretty universal, but the UK really loves the things. There's nothing on the packaging to specify that this is a Hot Topic exclusive, or that there's anything special inside: it's easy to identify the Cerberus mold by weight - it's by far the biggest and heaviest figure in Series 2 - but until you rip open the opaque green bag, you won't know whether it's the normal version or the variant.