The Mentors are an evolved form of amphibian native to Thoros Beta. They have no mobility on land, although it is possible they may be mobile in water. They need watering continually, and must be carried by pall bearers when on land.
Two things drive the Mentors of Thoros Beta: Money and Power. They ran the Galatron Mining Corporation, as well as other interplanetary businesses, some of which sold laser weapons to Thordon and Krontep. The sixth Doctor and Peri encountered the Mentors on the planet Thoros Beta where the future of the leader, Kiv, hung in the balance. Kiv's brain had grown to a point where his skull could not contain it. This would spell disaster for the Mentors' profits.
Crozier, a scientist funded by the Mentors, was in charge of transplanting Kiv's brain into another host. The Doctor was found to be unsuitable, but Peri was found to be perfect. Crozier took the decision to operate on Peri.
Due to the Valeyard tampering with footage of the events there are conflicting reports of what happened. One version of events is that the experiment was a success and Peri's mind was replaced by the mind of Kiv. This would have had a far reaching consequence for the future of the Mentors. When Peri's body aged, or was damaged, Kiv could be transplanted again - it meant Kiv could effectively live forever. For this reason the Time Lords used King Ycarnos as an assassin to kill Peri/Kiv.
Another version is that the experiment never took place, and Peri became a Queen living alongside King Ycarnos, and Kiv's condition presumably worsened causing his termination.