Dark Fox, the vile form of Fox McCloud, appears in the game Star Fox: Desperado. His appearance is that of the black clothed form of Fox in Super Smash Bros. Brawl, except the eyes are deep red. Throughout Desperado, Fox uses this form's various powers to acquire various Jewels of Rame, 1 of the 5 he starts out with. As you play, Dark Fox appears to slowly change Fox into him as the game progresses. In the final battle, Dark Fox has completely overtaken Fox and then turns himself into pure darkness, becoming Pure Dark Fox. In his fight with High Heija, he attempts to defeat her with a blast of dark energy when they are both sucked into the High Hieja's Banish Gate. Although he lives as Fox's opposite, he is in fact a separate entity channeling his power through fox for his own purposes. The game even hints that Dark Fox lives on the other side of the Banish Gate. This character, along with others, appears only in Star Fox: Desperado.
Note: this information is only a leak and does not confirm the games creation.