Nereus

In Greek mythology, Nereus (Νηρεύς) was the eldest son of Pontus (the Sea) and Gaia (the Earth), with Pontus himself being a son of Gaia. Nereus and Doris became the parents of 50 daughters (the Nereids) and a son (Nerites), with whom Nereus lived in the Aegean Sea.

Name
The name Nereus is absent from Homer's epics; the god's name in the Iliad is the descriptive ἅλιος γέρων Old Man of the Sea, and in the Odyssey the combination of ἅλιος γέρων and Πρωτεύς Proteus. Besides Nereus and Proteus, the descriptive "Old Man of the Sea" was also used for other water deities in Greek mythology, who share several traits; such as Phorcys, Glaucus, and perhaps Triton. It is suggested that the "Old Man of the Sea" had at one time played a cosmogonic role comparable to that of Oceanus, and could have received different names in different places. It is not known whether the name Nereus was known to Homer or not, but the name of the Nereids is attested before it, and can be found in the Iliad. Since Nereus only has relevance as the father of the Nereids, it has been suggested that his name could actually be derived from that of his daughters; while the derivation of the Nereids from Nereus, as a patronymic, has also been suggested. According to Martin Litchfield West (1966), Nereus is much less important than his daughters, mentioning that Herodotus offered "the Nereids, not Nereus, as an example of a divine name not derived from Egypt".

In Hesiod's Theogony, which is where the name was first attested, Nereus is presented in immediate juxtaposition to Eris; something that also extends to their children. First of all, there exists a feminine-masculine opposition. Also, Eris is the oldest and most important child of Nyx, while Nereus is Pontus' oldest and most important son. Another example is Hesiod's choice of verbs and adjectives used 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. This has prompted scholars to propose a derivation from Ἔρις Discord with the negative prefix νη  added to it; namely Ne-Eris Not-Discord, which evolved to Νηρεύς (< νη-ερ(ι)-ευς). Furthermore, Hesiod also played with the verbal likeness between Nereus and his last daughter Νημερτής Unerring, whose name also bears the negative prefix νη.

Another possible etymology could be from νηρόν, (fresh) water or fish, which is a contraction of the Greek adjective νεαρός,  new, fresh, young. It is commonly believed that the contraction of νεαρός to νηρός happened later than Hesiod; however, the contraction of ε and α to η is quite old, and is widespread over many Greek dialects.

The name could also be related to the Hesychian glosses νηρίδας hollow rocks or νηρόν low-lying. Robert S. P. Beekes (2010) favors a Pre-Greek (pre-Indo-European) origin, as is suggested by the suffix εύς,. Another view is that of Apostolos Athanassakis (1983), who suggested an Illyrian origin for the name, and compared it to the Albanian word njeri man.

According to August Fick (1890), the closest Indo-European relative of Nereus, as well as the Nereids, is the Lithuanian verb nérti to dive; also, the Lithuanian noun nėrõvė mermaid has been associated with the Nereids. Papachristophorou (1998) supported a derivation from the aforementioned Lithuanian verb, citing Pierre Chantraine (1968), while Tsantsanoglou (2015) considered the relation plausible.

The name of the Nereids has survived in modern Greek folklore as νεράιδες, fairies.

Mythology
In the Iliad, the Old Man of the Sea is the father of Nereids, though Nereus is not directly named. 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 who managed to catch him even as he changed shapes. Nereus and Proteus (the "first") seem to be two manifestations of the god of the sea who was supplanted by Poseidon when Zeus overthrew Cronus.

The earliest poet to link Nereus with the labours of Heracles was Pherekydes, according to a scholion on Apollonius of Rhodes.

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.

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."

Nereus was known for his truthfulness and virtue: "But Pontos, the great sea, was father of truthful Nereus who tells no lies, eldest of his sons. They call him the Old Gentleman because he is trustworthy, and gentle, and never forgetful of what is right, but the thoughts of his mind are mild and righteous."

The Attic vase-painters showed the draped torso of Nereus issuing from a long coiling scaly fishlike tail. Bearded Nereus generally wields a staff of authority. He was also shown in scenes depicting the flight of the Nereides as Peleus wrestled their sister Thetis.

In Aelian's natural history, written in the early third century, 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."

Nereus was father to Thetis, one of the Nereids, who in turn was mother to the great Greek hero Achilles, and Amphitrite, who married Poseidon.