Zog (The Ultimate Adventure)

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

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

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. (SP: Doctor Who: The Ultimate Adventure)

Other information
Zog was one of several monsters summoned by the Rani to fight the Doctor in various of his incarnations. (DW: Dimensions in Time)

Behind the Scenes
Jon Pertwee as the Third Doctor and later Colin Baker as the Sixth Doctor performed in Doctor Who: The Ultimate Adventure, with David Banks (the Doctor's understudy) performing as an original stage-only Doctor. Therefore, it remains unclear which Doctor "really" travelled with Zog.