Oyo Empire metropolitan Oyo. This may have been fine in the 15th century when Oyo controlled only its heartland. But to make and maintain farther conquest, the structure underwent several changes. Oyo maintained a semi-standing army of specialist cavalry soldiers called the "Eso", the "Esho" or, formally, the Eso of Ikoyi. These were 70 junior war chiefs who were nominated by the Oyo Mesi and confirmed by the Alaafin of Oyo. The Eso were appointed for their military skill without regard to heritage and were led by the Are-Ona-Kakanfo. After Oyo's return from exile, the post of Are-Ona-Kakanfo was established as the
Yoruba people Venezuela, Saint Lucia, Jamaica, Brazil, Grenada, and Trinidad and Tobago, and other countries. As an ethnic description, the word "Yoruba" (or more correctly, "Ya raba") was originally in reference to the Oyo Empire and is the usual Hausa name for Oyo people as noted by Hugh Clapperton and Richard Lander. It was therefore popularized by Hausa usage and ethnography written in Ajami during the 19th century by Sultan Muhammad Bello. The extension of the term to all speakers of dialects related to the language of the Oyo (in modern terminology North-West Yoruba) dates to the second half of the 19th