Two-Shape

In ancient Kuzugawan mythology, the Futatsokeijō (literally "two shape" in Kuzugawan), or better known in present times as the Two-Shape, is a legendary creature capable of mimicking the appearance of her next victims in order to lure them, and likewise confuse potential emancipators. Akin to that of a skin-walker, the Two-Shape's strength as well as some of her abilities changes depending on the type of skin, pelt, scale, hide, etc. she uses to cover her nape (e.g. A rat's fur will gain her the speed of a rat; the pelt of a bear grants her heavy strength). She cannot mimic higher beings such as Dragons, Great Eyes, Sages, or Fables.

Hence in ancient times, one reason why encountering one's doppelgänger was bad luck is because people thought the doppelänger was the Two-Shape in disguise.