This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Unhcegila (Lakota)
In Lakota mythology, Unk Cekula is a serpentoid creature which was responsible for many unexplained disappearances and deaths. Her male counterpart is known as Unk Tehi. She was described at first as having no real shape or form; she had eyes of fire, and a fanged mouth that was shrouded in a smoky or cloudy mass. As time went on further, her form was exposed as being massive, with a long scaly body whose natural armor was almost impenetrable.
Her eyes burned with wrathful hunger, her claws were like iron, and her voice raged like thunder rolling in the clouds. Whoever looked upon her will become blind or go insane. Her weakness is a seventh spot on her torso, behind of which her heart lies within, which burned fierily. To kill her, one has to shoot a medicine arrow at it.