Arthur (The Girl in the Fireplace)

Arthur was the name given by the Tenth Doctor to a grey horse which had wandered through a time window from 18th century France onto the SS Madame de Pompadour. The Doctor wanted to keep Arthur although Rose would not let him. He protested, "I let you keep Mickey!"

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. (TV: The Girl in the Fireplace)