"Al Khamsa" (الخمسة) is a designation applied to specific desert-bred bloodlines of the Arabian horse considered particularly "pure" by Arabian horse breeders, who sometimes also describe such lines with by use of the Arabic word asil, meaning "pure". It also refers to a mythical origin story of the breed
Al Khamsa roughly translates as 'The Five'. It refers to a mythical group of foundation mares that were the legendary founders of the Arabian breed. While some breeders claim these mares really existed, there is no objective, historical way to verify such a claim.
The modern definition of an Arabian as Al Khamsa usually refers to a horse that can be verified in every line of its pedigree to trace to specific named desert-bred Arabians with documentation that their breeding was attested to by a Bedouin seller who had sworn a formal oath (generally invoking Allah) that the animal was asil or pure of blood. This standard is only met by approximately two percent of all registered Arabians today. Such horses included the desert-bred imports of the Crabbet Arabian Stud, the imports from Syria of Homer Davenport, many of the horses imported from Egypt that were originally bred by Muhammad Ali of Egypt, Abbas Pasha, Ali Pasha Sherif, or the Royal Agricultural Society and its successor organizations, and other desert-bred horses obtained throughout the Middle East by buyers such as Carl Raswan who were familiar with bloodlines and the formal sales procedures of the Bedouin to properly document animals of Asil bloodlines.