The Teller are large muscular sentient humanoids gifted with powerful telepathic powers. They have grey elephant-like skin and elephant-like feet, a vaguely snail-like head with articulated eye stalks on each side of it, a short ribbed tail and two long claws serving as hands. Their nostrils are presumably situated under in series, along their eye stalks. One of their main defense mechanism consist of emissions of psywaves that can liquify the brain of their victims, indicently causing the skull to cave in.
They probably developed a primitive culture on thier howeworld but were all exterminated after being discovered by offworlders, presumably humans, at some point in the future. Two were kept alive to serve as guilt detectors for a galactic bank until the bank was closed and they were relocated to an uninhabited world.
Source: Doctor Who universe, Time Heist TV episode.