Rhesus was a Thracian king and an ally of king Priam of Troy. He was a son of Eioneus or the river-god Strymon and one of the Muses. His name seems to be a 'belligerent' one, from rhexis, 'breaking', so that he is a 'breaker'.

The king's name came to be applied, apparently with no logical connection at all, to the so called rhesus monkey. This monkey was widely used in medical experiments, and so passed its name in turn to such expressions as the 'rhesus factor' of blood.