In Greek mythology, Rhesus was a mythical king of Thrace who fought as a Trojan ally in Homer's Iliad. He arrived late with horses whiter than snow and golden armour, and was slain in his sleep by Diomedes during the night raid of Book 10 while Odysseus drove off the team. Later writers make him son of the river Strymon and a Muse; Euripides' Rhesus expands the episode and grants him a kind of immortal afterlife in a Thracian cave.
Adapted from Wikipedia, "Rhesus (king of Thrace)" · CC BY-SA 4.0
I.Family
Homer names his father Eioneus, perhaps linked to historic Eion at the mouth of the Strymon. Later writers give him a Muse mother — Euterpe, Calliope, or Terpsichore — and the river-god Strymon as father; Apollodorus records the Muse-catalogue dispute. Stephanus of Byzantium mentions a sister Sete, mother of Bithys by Ares; Conon names brothers Olynthus and Brangas. One account makes Heracles and Bolbe his parents instead.
II.The night raid
Rhesus reached Troy late because Scythians had attacked Thrace. Dolon, captured while spying for Hector, told Odysseus and Diomedes of the new Thracian camp and of Rhesus' matchless horses and godlike golden armour. While the Thracians slept, Diomedes killed twelve of them and Rhesus in his tent; Odysseus loosed the horses and drove them to the Achaean ships. Rhesus thus died without ever fighting by day — the ally killed on his first night.
The same action drives the tragedy Rhesus transmitted among Euripides' plays. There his Muse-mother arrives, blames Odysseus, Diomedes, and Athena, and announces that Rhesus will become immortal but dwell in a cave. Scholia and the play give him a more heroic stature than Homer's brief victim. Parthenius links the Thracian king with the Bithynian huntress Arganthone; a Bithynian river also bore the name Rhesus.
III.Reception
Ancient art shows Odysseus and Diomedes stealing the horses (for example a red-figure situla by the Lycurgus Painter, c. 360 BC). Rhesus Glacier in Antarctica and the Jovian asteroid 9142 Rhesus commemorate the Thracian king. This notice concerns the Trojan-War figure, not the river-god alone.