Name – Arthur
Species – Horse
Planet – Earth
Temperament – Well-trained
Appearances – Girl in The Fireplace

Arthur the horse wandered from the gardens of the Palace of Versailles into the space ship, through one of the time windows. The Doctor rode him through the last window, a grand mirror in the ballroom in order to defeat the Clockwork droids and save Reinette. Presumably Arthur remained in Versailles when The Doctor went home.