[1] They were originally a professional group of cavalry officers who became the elite corps of the Oyo army.
They evolved within a number of prominent families in Ikoyi, in the metropolitan province of Oyo in the 17th and 18th centuries, manifesting and sustaining high standards of bravery and proficiency, with their own culture and code of honour handed down from father to son.
With the collapse of the empire at the beginning of the 19th century, and the shift of the capital southwards into the forest zone, cavalry ceased to be the elite of Oyo's army, and the Eso of Ikoyi were dispersed to learn new arts of war.
Among generations of descendants, young men in several locations often called Ikoyi aspired to live up to the reputation of the Eso.
By the second half of the 19th century, Eso had become a title of honour that was conferred on individual younger warriors for outstanding achievement in battle, reminiscent of the bravery and code of honour of the classical Eso of Ikoyi.