According to McGonagall, true Seers are extremely rare. Sybill Trelawney is the only Seer portrayed in the books, and is considered an "old fraud" by her students -- although it is mentioned that Sybill's great-great-grandmother, Cassandra Trelawney, was a renowned Seer in her day. Trelawney is ultimately sacked by Dolores Umbridge in the fifth book for her lack of ability. She has, however, twice made true prophecies -- both of significance to Harry Potter -- but had no recollection of either prediction afterward.