This article explains some of the history behind eating horse meat, as well as the reasons some countries have horse meat taboos.
Here's the intro:
Horse meat is the culinary name for meat cut from a horse. It is a major meat in only a few countries, notably in Central Asia, but it forms a significant part of the culinary traditions of many others, from Europe to South America to China. The top eight countries consume about 4.7 million horses a year. For the majority of mankind’s early existence wild horses were hunted as a source of protein. It is slightly sweet, tender, low in fat, and high in protein. However, because of the role horses have played as a companion and as a worker, it is a taboo food in many cultures. These historical associations, as well as ritual and religion, led to the development of the aversion to the consumption of horse meat. The horse is now given pet status by many in the western world, which further solidifies the taboo on eating its meat. This avoidance (and the loss of taste for it) is relatively modern, although it arises out of complex historical and cultural origins.