The Threarah was the Chief Rabbit of Sandleford Warren. His name in Lapine, threar, means "rowan tree." It is believed he was called the Threarah because there was only one rowan tree in the vicinty of the warren.
He was the uncle of Silver. He ignored Fiver's warning to evacuate the warren, and consequently, he is among the many rabbits of the warren who perished. In the film, the Threarah was gray furred, old (like in the novel), and rather obese. In the film he wasn't given a name- only credited "The Chief Rabbit". He was played by Ralph Richardson.