The Giant's Giant Cat is an original character from the Land of Stories series. He is a pet of the Giant from the Beanstalk and appears first in The Enchantress Returns, ch 19. By his description and the author's known love for his own cat (who is also listed in the book's acknowledgements), it is speculated that this is a little cameo for Chris Colfer's own pet, Brian.
Appearance and Personality
"The giant's cat was obese, with gray fur, black stripes, and white paws. It blinked its large green eyes slowly and stared down at them in a flirtatiously fatal manner. A red band was wrapped around its neck, and a bell the size of one of their heads swung from it."
The cat has been living alone in the castle since the giant died, but has been feeding himself well, going by the bird carcasses littering the castle floors. When Jack, Goldilocks, Froggy and the twins enter the castle, he attacks them and tries to follow them down the Beanstalk. Eventually it takes Red chopping at the beanstalk and Clawdius barking at it to make him retreat back into the cloud castle.
In Beyond the Kingdoms, the giant cat makes a return. When Alex ends up in the giant's castle and meets Mother Goose there, it turns out the cat is still living there too. Mother Goose has put him on a diet and called him George Clooney.