Referring to their appearance, demons can take any desired appearance, even that of an angel of light (2 Corinthians 11:14). Nevertheless, they were generally described as ugly and monstrous beings by Christian demonologists, without any valid reason. Many of these descriptions are almost (if not completely) ridiculous, but have inspired famous painters like Luca Signorelli, Hieronymus Bosch, Goya, the artist that made the drawings for the Dictionnaire Infernal, and others.
Some early theologians described demons with the appearance of dirty Ethiopians.
But according to Nicholas Remy, the figure of the demons is imperfect, which he deduced from the descriptions given by those accused persons interrogated during the witch trials; he wrote "that proves how marvellous God's love is, even for the most miserable human beings, being that demons can never take a human figure in a perfect form, and so the most stupid people are able to discover them".
The idea that demons have horns seems to have been taken from the Book of Revelation 13:1 (here seems that John was inspired by Leviathan) and 13:11. The book of Revelation seems to have also inspired some absurd depictions of demons (Revelation 13:1-2). This idea can also been associated with the depiction of certain ancient gods like Baal, Moloch, the shedu, etc, which were portrayed as bulls, as men with the head of a bull, or wearing bull horns as a crown.
Nothing explains the idea that the Devil could be represented with a tail, but the trident attributed as his "sceptre" might have been inspired by an early contact of Christians with Hindus (that contact is registered in history and still exist some communities of Christians that follow the ancient rite of apostolic times in India), because the trident is Shiva's weapon, and that god is considered by most followers of Hinduism, the Vaishnavas, worshippers of Vishnu as main deity (with the exception of the minority of the Shaivas, Shiva's worshippers) as the destroyer, Kali's husband, comparable although not equal to the Devil.