Tolkien’s illustrated story about a dog named Rover is titled Roverandom.
Tolkien wrote Roverandom in 1925 after his young son Michael lost a favorite toy dog during a holiday at the seaside. The story follows Rover, a transformed or enchanted dog who experiences adventures involving the Moon and the sea. Tolkien illustrated the manuscript himself, creating pictures to accompany the tale.
Roverandom was not published during Tolkien’s lifetime. It appeared in 1998, edited by Christina Scull and Wayne G. Hammond. The work is separate from Middle-earth and is best understood as a children’s fantasy connected to an actual family incident, rather than as a novel in The Hobbit or The Lord of the Rings sequence.