Arthur (The Girl in the Fireplace)

Arthur was the name given by the Doctor to a white horse which had wandered through a time window from 18th century France onto the SS Madame De Pompadour. When the Clockwork Droids sealed off the time window to 1758 to keep the Doctor out, he used Arthur to break through the seal and rescue Madame de Pompadour. (DW: The Girl in the Fireplace)