[2] Besides Nereus and Proteus, the descriptive "Old Man of the Sea" was used for other water deities in Greek mythology who share several traits, among them Phorcys, Glaucus, and perhaps Triton.

Hesiod chooses verbs and adjectives to describe Nereus in juxtaposition to Eris' children, such as ἀ-ψευδέα 'does-not-lie' and ἀ-ληθέα 'does-not-forget', as opposed to Ψευδέα 'Lies' and Λήθη 'Forgetfulness'.

[7] This has prompted scholars to propose a derivation from Ἔρις Eris 'Discord' with the negative prefix νη‑ ne‑ added to it; namely, Ne-Eris 'Not-Discord', which evolved to Νηρεύς (< νη-ερ(ι)-ευς).

He was never more manifestly the Old Man of the Sea than when he was described, like Proteus, as a shapeshifter with the power of prophecy, who would aid heroes such as Heracles[17] who managed to catch him even as he changed shapes.

[18] During the course of the 5th century BC, Nereus was gradually replaced by Triton, who does not appear in Homer, in the imagery of the struggle between Heracles and the sea-god who had to be restrained in order to deliver his information that was employed by the vase-painters, independent of any literary testimony.

[19] In a late appearance, according to a fragmentary papyrus, Alexander the Great paused at the Syrian seashore before the climacteric battle of Issus (333 BC), and resorted to prayers, "calling on Thetis, Nereus and the Nereids, nymphs of the sea, and invoking Poseidon the sea-god, for whom he ordered a four-horse chariot to be cast into the waves.

In Aelian's natural history, written in the early third century,[23] Nereus was also the father of a watery consort of Aphrodite and lover of Poseidon named Nerites who was transformed into "a shellfish with a spiral shell, small in size but of surpassing beauty."