[Lachwen]

I've always believed that Gothmog was, as Boromir88 said, a Black Númenórean. As for the name...well, it said in the books that the Mouth of Sauron had been in Sauron's service for so long he had forgotten his real name, so, if Gothmog is of his kin, the same thing has probably happened. My guess is that Gothmog was a name given to the Lieutenant of Morgul, as a sign of prestige and power - rather like the battering-ram used against Minas Tirith being called Grond.