Shop our official range of The Gruffalo Costumes at Smiffys. The Gruffalo is a children's book written in rhyme by Julia Donaldson in 1999, and tells the story of a mouse taking a walk in the woods. On his walks, the mouse warns off his enemies using an invented friend called a "gruffalo". Much to his surprise, the mouse then meets the actual Gruffalo! Perfect for Book Day, our kids gruffalo costumes are great for a storybook themed outfit.