The demon Bifrons appears in the grimoires Book of Incantations, The Discoverie of Witchcraft, Pseudomonarchia Daemonum, and The Lesser Key of Solomon
From The Lesser Key of Solomon, Crowley/Mathers edition:
The Forty-sixth Spirit is called Bifrons, or Bifrous, or Bifrovs. He is an Earl, and appeareth in the form of a Monster; but after a while, at the Command of the Exorcist, he putteth on the shape of a Man. His Office is to make one knowing in Astrology, Geometry, and other Arts and Sciences. He teacheth the Virtues of Precious Stones and Woods. He changeth Dead Bodies, and putteth them in another place; also he lighteth seeming candles upon the Graves of the Dead. He hath under his Command 6 Legions of Spirits. His Seal is this, which he will own and submit unto, etc.