In Greek mythology, Hermus (Hermos) is the god of the Lydian river Hermus — the modern Gediz — a major stream of western Anatolia. Like most river-gods he is son of Oceanus and Tethys in Hesiod; Homer knows the eddying Hermus beside the Gygaean lake. Distinct from Hermus the Aegyptiad bridegroom slain by his Danaid wife, and from the Attic companion of Theseus.
Adapted from Wikipedia, "Hermus" · CC BY-SA 4.0
I.River-god of Lydia
Hesiod names Hermus among the rivers born to Oceanus and Tethys. As potamos of Lydia he belongs to the Anatolian brotherhood of waters that frame Homeric geography: the Iliad sets a Trojan ally's birth by the eddying Hermus and the Gygaean lake.
Later mythographers call him father of Lydian nymphs. The living river drains the Aegean face of Lydia toward the gulf near Phocaea — a real landscape under the god's name.
II.Homonyms
Wikipedia's Hermus index also lists an Egyptian prince, son of Aegyptus by Caliadne, murdered on his wedding night by the Danaid Cleopatra; and an Athenian noble left by Theseus at Pythopolis, whose name was later confused with Hermes.
The atlas id hermus is reserved for the Lydian river-god — not the Aegyptiad or the Attic companion.