Variant werewolves
No one knows what causes one to be change-hearted; it doesn't run in the blood, or wax and wane wth the moon, or be predictable in any way… other than in the scent that we can catch when near one as-yet-unchanged.
And even then, the Wyld's slipperiness manifests: a wolf's bite is as likely to birth a were-raven or were-bull as another wolf, since the change that takes hold flows along the paths of the new changer's heart, rather than from the one who decided to change them.