The Paradise of Death
Naglons were a humanoid species. They were part of the Federation led by Parakon.
Their skin was purple and saggy, covered with warts and pustules. Their eyes and mouth were in the same place as Earth humans.
Tragan was a Naglon, and used a mask to disguise his alien nature while on Earth. The Doctor had encountered Naglons before meeting Tragan. (The Paradise of Death)