From Punters in Australia comes this story of a tale from the 1920s and 30s:
Much of the greyhound jockey craze originated in California, having been the brainchild of Loretta and Charles David. The couple imported twelve baby capuchin monkeys from Panama at great expense, before raising them and socializing them in the company of greyhounds. Once they had bonded with their canine pals, the monkeys were painstakingly taught to ride the dogs.
The monkeys took to the racing with such relish that they actually became highly competitive beasts during races, even being known to use their own tails as an in-built whip to urge an extra effort out of their mount!
Bananas and peanuts were always on hand to reward the winning hoops, so there was incentive enough not to hook one come raceday! Capuchin monkeys are, however, considered the most intelligent of the New World monkey species, so who's to say a few weren’t ‘got at’ by any enterprising race-fixers out there…
The monkey jockey phenomenon was very well received across the United States, and indeed eventually found its way to the dog tracks of Australia, including at Newcastle and Mascot. Aussie promoters even added hurdles and water-jumps for dog and monkey-rider to negotiate.