The demon Vepar appears in the grimoires Book of Incantations, The Discoverie of Witchcraft, The Book of the Office of Spirits, Pseudomonarchia Daemonum, and The Lesser Key of Solomon
From The Lesser Key of Solomon, Crowley/Mathers edition:
The Forty-second Spirit is Vepar, or Vephar. He is a Duke Great and Strong, and appeareth like a Mermaid. His office is to govern the Waters, and to guide Ships laden with Arms, Armour, and Ammunition, etc., thereon. And at the request of the Exorcist he can cause the seas to be right stormy and to appear full of ships. Also he maketh men to die in Three Days by Putrefying Wounds or Sores, and causing Worms to breed in them. He governeth 29 Legions of Spirits, and his Seal is this, etc.
Timeline of related events
1536Publication of De praestigiis daemonum (lists this demon)
1563First known appearance of The Book of the Office of Spirits (lists this demon)
1584Publication of The Discoverie of Witchcraft (lists this demon)
1641Publication of the Lesser Key of Solomon (lists this demon)