Hesiod gives Erytheia its oldest surviving clear role in the Geryon story. Chrysaor and the Oceanid Callirrhoe become parents of three-headed Geryon, whom Heracles kills in sea-girt Erytheia after crossing Ocean and killing Orthrus and the herdsman Eurytion. The island is therefore defined first by remoteness, cattle and the western boundary of heroic travel rather than by measured geography.

The name is naturally associated with redness or the glow of the west, and later tradition strengthens that image by giving Geryon red or crimson cattle. Pseudo-Apollodorus explicitly calls the herd red and identifies Erytheia as an island near Ocean now called Gadira. The colour of the herd belongs to this later mythographic description rather than to every early source.

Heracles reaches the island only after an extraordinary journey. After crossing Libya and Tartessus, he threatens Helios with his bow, and the Sun gives him a golden goblet in which he can cross Ocean. On Erytheia Orthrus detects the intruder and is killed, Eurytion follows, and Menoetes warns Geryon. Heracles then kills Geryon and removes the cattle, ending the island's role as a guarded western pasture.

Herodotus places Erytheia near Gadeira outside the Pillars of Heracles, and later writers debate how exactly the island fits the Cádiz archipelago. Strabo preserves competing identifications around Gades and Tartessus, while Pliny records the names Erythia, Aphrodisias and the Isle of Juno for islands in the group and also reports a rival location opposite Lusitania. These accounts show ancient geography trying to anchor an older myth rather than proving that one specific island was the original Erytheia.

Strabo also records extraordinarily rich pasture near Gades, where cattle became so fat that herders had to manage their condition carefully. Such observations made the western landscape seem well suited to the famous cattle story. They are later geographical explanations for why the myth might belong there, not evidence that a historical herd of Geryon once occupied the island.

A separate myth also puts Helios's cattle on Erytheia when the giant Alcyoneus drives them away before the Gigantomachy. Those cattle are not the same herd as Geryon's in the principal traditions. The repeated association of Erytheia with divine or extraordinary cattle reinforces its identity as a distant pasture at the border between human geography and myth.