The ostrich was a large, flightless bird native to Earth.
Prior to or during the 1890s, Inspector Quick saw an ostrich egg at a museum. (AUDIO: Jago & Litefoot & Patsy)
After his imprisonment at the Prison, the Twelfth Doctor jokingly renamed Clara Oswald "Clara Ostrich", due to her having a long neck and furthermore claiming that neither of them liked the name 'Oswald'. He then simply referred to her as "Ostrich". (PROSE: The Blood Cell)
By the year 5,000,000,000, all that remained of this species was an egg in the possession of Lady Cassandra, which actually contained a feed used to enhance her teleport signal. Going by legend, Lady Cassandra said that the ostrich had a fifty-foot wingspan and could shoot fire out of its nostrils. (TV: The End of the World)
Behind the scenes
According to non-narrative source Monsters and Villains, the ostrich became extinct in 2051.