This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Ur (Mandaean)
In Mandaeism, ?Ur is the king of the World of Darkness or underworld. He is the son of Ruha, the queen of the underworld, and her brother Gaf (also spelled Gap), one of the giants in the World of Darkness described in book 5 of the Ginza Rabba. Ur is typically portrayed as a large, ferocious dragon or snake.
He is represented by the image of a serpent on the skandola talisman. The Mandaean Book of John contrasts Ur, the King of Darkness, with the King of Light (Hayyi Rabbi). Ur has also been referred in Mandaean texts as Leviathan. According to the Right Ginza 5.1, his mother Ruha called him "the Great Giant, the Power of Darkness"