The Swanmay is a fantasy creature (mostly from the D&D world).
They are attractive human females who have the ability to turn in to giant swans. In their human form, they resemble an attractive woman in her mid 20's (usually wearing light armor, and carrying forest style adventuring gear). Someone with a shrewd eye may notice that they all wear some sort of jewelry with a couple swan feather on it (this token is the source of their power). By simply grasping the token, the woman will be transformed into a swan (these tokens only function for their original owners, no one else can use them).
Unlike other lycanthropes, the swanmays have full control over all their transformations, and all of them voluntarily chose to become swanmays (you have to be invited by a swanmay group, and then undergo a secret ceremony).
All swanmays are members of a secret sorority, dedicated to the protection of the forests. They usually live communally in well built lodge style house near a water source (these homes will usually have multiple secret exits). The surrounding area will usually be home to an abundance of swans and other waterfowl (and may also be littered with the dead bodies of evil forest creatures, or poachers). They will usually be friendly with the local elves, druids, dryads, and other forest creatures. They will not tolerate any sort of evil humanoids in their forests (and are quite wary of any and all human strangers as well).
A swanmay is equally combat effective in human or swan form. They can attack like any normal human when not transformed. Once transformed they can only be damaged by enchanted weapons, but have limited attack options.