Arthur was the name given by the Tenth Doctor to a random 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!" (TV: The Girl in the Fireplace)
Becoming irritated after Rose and Mickey wandered off despite his warnings, the Doctor confided his frustrations in Arthur as he indulged in an attempt at reverse psychology, telling the horse that he was free to roam the ship. Arthur, however, did not respond, and the Doctor agreed that he was not convicing. (COMIC: The Road To... )
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)
Behind the scenes
Arthur appeared as a character in the video game LEGO Dimensions.