One who has become unscrupulous or irresponsible due to the sudden acquisition of wealth. The phrase alludes to several related proverbs, such as "give a beggar a horse and he'll ride it to death" and "set a beggar on horseback and he'll ride to the devil," which suggest that an unexpected windfall is often misused or squandered. I'm not surprised to hear that he's broke again—he always seemed like a beggar on horseback.