* The Discovery: The first detailed description of this monkey species was published in 1776 by the German naturalist, Johannes van de Laar. He encountered them in the Indian subcontinent, particularly near the Indus River.
* The Naming: The Indus River in ancient Greek was known as "Indus". In ancient Greek, the river was also sometimes called "Rhesus".
* The Connection: Van de Laar, being a scholar of classical languages, likely chose to name the monkey after the river where he found them.
Therefore, the name "Rhesus" for the monkey reflects the geographical location of its discovery.