Nereb (reversal of the name Beren) was the orc-name that Beren took upon himself when in that disguise during the Quest for the Silmaril. His companion, Finrod Felagund, called himself Dungalef.
J.R.R. Tolkien, Christopher Tolkien (ed.), The Lays of Beleriand, "III. The Lay of Leithian: Canto VII (Beren and Felagund before Thû)"