The demon Furfur appears in the grimoires Book of Incantations, The Discoverie of Witchcraft, The Book of Spirits, Pseudomonarchia Daemonum, The Lesser Key of Solomon, and Dictionnaire Infernal
From The Lesser Key of Solomon, Crowley/Mathers edition:
The Thirty-fourth Spirit is Furfur. He is a Great and Mighty Earl, appearing in the Form of an Hart with a Fiery Tail. He never speaketh the truth unless he be compelled, or brought up within a triangle A. Being therein, he will take upon himself the Form of an Angel. Being bidden, he speaketh with a hoarse voice. Also he will wittingly urge Love between Man and Woman. He can raise Lightnings and Thunders, Blasts and Great Tempestuous Storms. And he giveth True Answers both of Things Secret and Divine, if commanded. He ruleth over 26 Legions of Spirits. And his Seal is this, etc.
Timeline of related events
1536Publication of De praestigiis daemonum (lists this demon)
1584Publication of The Discoverie of Witchcraft (lists this demon)
1641Publication of the Lesser Key of Solomon (lists this demon)