The Conduit

 was a sentient machine.

The Conduit had need of the resources of the Lakhothan planet. It was standard practice for the Conduit to change any "sentient animates" into plants and animals to minimise resistance. The Conduit changed the Lakhotha into plants and animals during the time the First Doctor, Steven Taylor and Katarina visited the Lakhotha tribe.

After returning to the TARDIS, the Doctor went in search of the cause of the transformations. He met with the Conduit and found that they had no remorse for their actions. However, the Doctor couldn't deal with the Conduit, as his priority was saving Steven from his dangerous injury. (PROSE: Scribbles in Chalk)