Trantis was a representative of his home galaxy when he allied himself with the Daleks in their plan to conquer the Solar System. When the Daleks got back their taranium core from the Doctor, they tested their Time Destructor on Trantis. When the device did not kill him, they discovered that the Doctor had given them a fake core. They then exterminated Trantis.
While publicity photos and some telesnaps have shown Trantis with tendrils hanging from his face, Day of Armageddon (the only surviving episode of The Daleks' Master Plan to feature Trantis) shows Trantis looking like a normal human, except with jagged, sharpened teeth.