The Theravada Standpoint
According to the Theravadin Vinaya, during the time of the Buddha, there was a layperson, a village head, who had many hunters working as his subordinates. Before he was enlightened, the hunters used to offer him large amount of meat from their hunt. After receiving some teachings from Shakyamuni Buddha, he eventually attained realization of the Theravada path of seeing and stopped eating meat. However, his subordinates continued to hunt and offer him meat. He would instead offer the meat to the monastics whenever they came begging for alms. Once the monastics ate the offered meat.
Some non-Buddhists then began to attack them by saying, “Laypersons would not eat that meat, but the disciples of Shakyamuni Buddha would. This is outrageous!” On hearing this, some bhikshus, seeking the Buddha’s advice, asked, “What should we do about these comments from others now that we are eating meat?” The Buddha then set the rule of eating only the three kinds of clean flesh of which some special requirements were also laid down. That is, the meat of snake, dog, horse and ox were not to be eaten even if they had met the standards of the three kinds of clean meat. Because Indians, during the time of the Buddha, considered the meat of these animals unclean like human’s. To date, the Southern Buddhist tradition still upholds this rule.
~ Depicted from Luminous Wisdom Book Series : Why Vegetarian?