Were-Cerberus

Appearance

The were-cerberus is a werewolf with three heads, and unlike the traditional shapeshifting lycanthrope, it cannot change shapes. It is permanently a nine foot tall humanoid beast, bristling with hair and claws. The three heads are not exactly friendly to each other and will snap and snarl at each other when the beast isn't focused on hunting or combat.

Special Abilities

Chorus of Fear - the werecerberus can release a discordant triple howl that causes supernatural fear in those who hear it. While a stalwart band of heroes would probably have heart to stand their ground, but their squires, retainers, non-supernatural mounts and horses are going to panic and flee.

Rending Bite - having three mouthfuls of teeth, the werecerberus can grab a foe with two mouths and forcing the foe into an exposed position for a lethal bite. Any foe who is bound in a grappling attack with the werecerberus runs the risk of the rending bite. A failed saving throw means the werecerberus inflicts maximum damage with their third bite. If they make a critical fail on the check the werecerberus delivers a throat ripping bite that kills the character. For those not wanting such a quickly lethal foe, the foe who ends up in this position is rendered prone and has 1D6 rounds before they bleed out. If a healer reaches them in time, they can be healed with magic and will only have a horrific scar from the incident.

Lord of Wolves - the werecerberus can release a magical howl that summons a pack of snarling wolves to it's aid. These wolves are large and aggressive. bite bite bite

Origin

The Werecerberus was created by a dark sorcerer (tm) who has a fascination with dark magic and creating horrific monsters to keep intruders and interlopers out of the Dark Forest (tm). The creature was created by fusing a normal human werewolf with a dire wolf, and with a 'Spirit of the Hunt' to create a tripartite monster that only knows fury and hunger and desolation.

It shouldn't be a real surprise that the werecerberus slaughtered the Dark Sorcerer who created it and devoured their corpse in a fit of fury.

Plot Hooks

Guardian of a Hidden Secret super important Treasure in the Dark Forest

The Werecerberus has secret knowledge and the PCs have to find a way to calm the creature down enough to speak with it.

An Unexpected Guardian - rather than a trope monster in the woods, the werecerberus has become a ranger/druid of the wood and now protects it and keeps it as a haven for the misshapen monsters created by other evil magic users. As such, it would have more of a greek mythology or World of Warcraft vibe, and less horror monster.