From the dust jacket:

Kickapoo is a Kansas mule with more brains than any mule in the whole state and more personality than a dozen mules from any state. Howdie was pretty well pleased with his mule until he read the posters about the Pony Express Race. Because who ever heard of a mule running in a pony race?

Howdie decided he had to have a pony and Kickapoo must go. So Howdie put an advertisement in the post office and waited for a buyer for Kickapoo. When the first buyer arrived Kickapoo was nowhere to be found. He laughed the second buyer off the farm, and he ran away from the third one. And then Kickapoo really took over and proved, to Howdie’s delight, that a mule was as good as a pony any day in the week.