Along the same lines as the pixie and minotaur being rare, limited, classes… here’s another:

Werewolf: The werewolf masqarades as a human except during full moons, during which time they transform into a werewolf. The wolf form has a bloodthirst type affect running constantly which forces them to attack everything they see. The form itself grows stronger with rank, running roughly on par with a good utility form of equivalent rank. When in form the werewolf looses all abilities he holds in his human form.

Classes allowed: All
Alignment: All
Ethos: All
Stats: Str 20; Int 20; Wis 20; Dex 20; Con 20; Chr 20
Size: Normal
Abilities: None
Vulnerabilities: None
Resistances: None

Inherent abilities:
Focus: Focuses the will of the werewolf on not transforming. This allows the werewolf to delay his transformation for an hour or two. When already transformed, this skill can allow the wolf to delay his transformation back to human similarly.
Bite: When in form, allows the werewolf to bite his foe and impart disease into them.
Quick Recovery: When in form, the werewolf heals extremely quickly.

...Rome...