Zog (The Ultimate Adventure)

Zog was a companion of the Doctor's, though not as innocent as he seemed.

Biography
As a slave, he served at the Bar Galactica run by Madame Delilah. When the Doctor, Jason and Crystal arrived, they found Karl and his mercenaries there waiting for them. Crystal formed a friendship with Zog and the three took Zog with them when they fled the bar. Zog continued to travel with them. (BFA:The Ultimate Adventure)

Zog was secretly an Aldeberian tyrant planning to enslave the universe. (ST: Face Value)

Behind the scenes

 * Zog was one of several monsters summoned by the Rani to fight the Doctor in various of his incarnations. (DW: Dimensions in Time)
 * Because Doctor Who: The Ultimate Adventure was a stage play which featured at various times the Third Doctor, the Sixth Doctor and a unique stage-only Doctor, which Doctor Zog "really" travelled with is unclear.
 * The audio adaptation of this story and the sequel short story feature Zog travelling with the Sixth Doctor, clearing up the question so far as BFA and PDA continuity are concerned.