The Zaross were a fictional race, created and used as a disguise by the Forzell, so they could be part of an intergalactic reality show, where the main race would invade a new planet every episode.
The Forzell created the Zaross so that they would be taken more seriously by the various other planets and species of the galaxy. They used heavy makeup to hide their true appearance.
Under the makeup, the Zaross had the appearance of a bipedal warthog, with large tusks. Not yet aware of them being a fictional species, the Tenth Doctor thought that they looked very close to the Forzell, not realising that the Forzell were the Zaross. (AUDIO: Infamy of the Zaross)