Werefox
Werefoxes are an obscure species of therianthrope native to the cosmology of Dungeons & Dragons, defined by their ability to transform partially or wholly into foxes. In fact, there are actually two distinct species of werefox; the Common and the Mystaran. It's also a term used for Kitsune in Werewolf: The Apocalypse.
Common Werefox
The "Common" werefox (a term only used here to distinguish it from its Mystaran counterpart) is also known formally as the Foxwoman and the Vixen; it is a strain of therianthropy only communicable to women, and is formally portrayed as the elven counterpart to the werewolf. In their "human" form, foxwomen appear as exceedingly beautiful silver-haired elf-maids; they can assume the form of silver foxes (a kind of magical silver-furred fox native to fey-touched or elf-inhabited forests that moves tracelessly through the wilderness) or surprisingly beautiful silver-furred anthropomorphic vixens.
The beauty thing isn't a joke. Foxwoman regeneration is so potent it actually causes scars and other blemishes to fade away month by month until they vanish into nothing. That's not the weirdest aspect of foxwoman biology. The "elfin" part of their curse is as infectious as the fox part. What does this mean? It means that any woman infected with this form of therianthropy (originally, only humans, elves and half-elves were susceptible, but in 3e any humanoid can be afflicted) will find themselves changing irrevocably into elf-maids, complete in every detail! Another biological oddity is that foxwomen are sterile; uniquely amongst therianthropes, they cannot bear children, and instead rely on spreading their curse to propagate.
Unlike most therianthropes, foxwomen are little inclined towards brutal rampages. Self-serving, vain and hedonistic, foxwomen only care about appeasing their desire for luxuries and comfort. Between their beauty, their great control over their feral instincts, their bewitching gaze and their propensity to train as bards or enchanters, they have little trouble attaining what they want.
Foxwomen debuted in Advanced Dungeons & Dragons 1st edition in the Monster Manual II - however, a prototype version, more overtly connected to the Kitsune appeared for OD&D in White Dwarf #19. They gained a patron goddess called Eshebala in Monster Mythology, and were converted to 2nd Edition in first the Monstrous Compendium and then in the Monstrous Manual. In Dungeons & Dragons 3rd Edition they would not be converted till Ravenloft Gazetteer I.
Precisely why it was Ravenloft 3e that brought the foxwomen back, who can say? Although foxwomen are common in the Demiplane of Dread, they are found in other settings, including the Forgotten Realms, with the splatbook "Elminster's Ecologies" saying they are particularly common in Cormyr, Sembia and the Dalelands.
Mystaran Werefox
On Mystara, werefoxes are basically a vulpine equivalent to the common werewolf, lacking the gender restrictions or elven traits of their counterparts from other worlds. Believed to descend from a Thyatian Rake named Leo Variantia, werefoxes are far more docile in temperament than their lupine counterparts (BECMI Neutral to their BECMI Chaotic); a werefox on the "rampage" is basically out to pull pranks and steal chickens, not kill anyone. They're mostly harmless tricksters and mischief-makers, infatuated with their own cleverness but typically not out to hurt anyone.
This version appeared in the Creature Crucible PC4: Night Howlers, which even provided rules for playing a werefox PC. In Dungeons & Dragons 3rd Edition Lycanthrope can now be applied to any animal, though only small races can become werefox due to size requirements.