According to the Hinayana tradition, one is allowed to eat only the ‘three kinds of clean flesh.’ Other kinds of meat are strictly forbidden. Now in Thailand and other Southeast Asian countries, the Sangha still upholds this practice. They think that not eating meat at all is to follow the decree of Devadatta. And the practice of eating the three kinds of clean flesh is rather in keeping with the precepts taught by the Buddha in the Theravadin Vinaya.
[Excerpt from Luminous Wisdom Book Series 1: The Right View ~Why Vegetarian?]