Said to be the mother of Gothmog, this ogress was not mentioned again by Tolkien in his later works. However, it is speculated that MERP/ICE used her name along with other Lost Tales names in order to create stats and biographies for their game. An example of this would be Lungorthin, the white balrog. It should be noted that Fluithuin mirrors a lot of traits of Lilith, Medusa, or Grendal's Mother. It also should be noted that there are many Maia that follow Melkor that are not talked about in the legendarium--Fluithuin could still be one of them even knowing her name did not appear in any of the published legendariums, though her race, the ogres, do appear in the published Hobbit. It also should be noted that Tolkien dropped the CONCEPT of Valar having children, this by no means mean that they couldn't have children (as demonstrated by Ungoliant and Melian, both spirits). So plausible use for Fluithuin is that she was a Maia infatuated with Melkor, sought to have a kid with him, violated the ban by Eru, and thus was cursed, made barren, and transformed into a hideous Ogre. Thus she could not reproduce with Melkor.