He is a big horse, tall and bony, with long legs and large knees and feet. His tail is short and scraggly. He's a proud animal, and in the Land of Oz, the respect and fairness shown to him make him a little arrogant.
Jim was born in Kentucky and was a race horse, defeating all who ran against him. In his later years he pulled a cab in Chicago until he finally retired to Hugson's Ranch.
One day Zeb Hugson drove him to Hugson's Siding to pick up Dorothy Gale. On their way back to the ranch, an earthquake struck and the entire carriage fell into the Earth, Jim and all. They lighted in the Land of the Mangaboos where Jim discovered that he could talk. From there, they traveled through the Valley of Voe, the Land of Naught, and a den of Dragonettes. When they came to a dead end, Dorothy had them transported to the Emerald City.
There, Jim met the Sawhorse and challenged him to a race to prove who was the better horse. The Sawhorse won, much to Jim's chagrin. Soon after Jim and Zeb asked to return home to San Francisco. (Dorothy and the Wizard in Oz)