Porridge is an original character from the Land of Stories series. She is Goldilocks' horse. Her name is probably derived from the classical fairy tale of Goldilocks and the Three Bears, in which Goldilocks eats from the bears' porridge. Porridge first appears in The Wishing Spell, ch 6.
Appearance and Personality
"A cream-coloured horse slowly traveled into the clearing. It picked up its hooves in a particular manner, as if it had been trained to tiptoe." [TLOS I, ch 6, p. 119]
Porridge is female, can defend herself in battle, and is said to be "the fastest horse in all the kingdoms".
In The Enchantress Returns, it is implied that she has a "complicated history" with a horse from Queen Red Riding Hood's stables, Buckle, and it is revealed at the end of the book that she is pregnant, with Buckle as a likely father.