Argus (or Argos) Panoptes was a hundred-eyed giant of Argos in the Peloponnese.
Once when Zeus was consorting with the Nymph Io, Hera suddenly came upon them. The god quickly transformed his lover into a white heifer. The goddess was not deceived and craftily demanded the animal from her husband as a gift. She then appointed Argos Panoptes to guard the heifer.
Zeus sent Hermes to rescue his lover. The god first tried to lull the giant to sleep with his music, but failing that, despatched him with his sword. It was from this endeavour that he earned the title Argeiphontes (the slayer of Argos). Hera nevertheless rewarded Argos for his service by placing his hundred eyes on the tail of the peacock, her sacred bird.
His parentage is stated differently, and his father is called Agenor, Arestor, Inachus, or Argus, whereas some accounts described him as an Autochthon. (Apollod. ii. 1, 2, &c.; Ov. Met. i. 264.)
He derived his surname, Panoptes, the all-seeing, from his possessing a hundred eyes, some of which were always awake. He was of superhuman strength.
after he had slain a fierce bull which ravaged Arcadia, a Satyr who robbed and violated persons, the serpent Echidna, which rendered the roads unsafe, and the murderers of Apis, who was according to some accounts his father, Hera appointed him guardian of the cow into which Io had been metamorphosed.
(Comp. Schol. ad Eurip. Phoen. 1151, 1213.) Zeus commissioned Hermes to carry off the cow, and Hermes accomplished the task, according to some accounts, by stoning Argus to death, or according to others, by sending him to sleep by the sweetness of his play on the flute and then cutting off his head. Hera transplanted his eyes to the tail of the peacock, her favourite bird. (Aeschyl. Prom. ; Apollod. Ov. ll. cc.)
"Chorus [of Danaides]: Is there a report that once in this land of Argos Io was ward of Hera's house? King: Certainly she was; the tradition prevails far and wide. Chorus: And is there some story, too, that Zeus was joined in love with a mortal? King: This entanglement was not secret from Hera. Chorus: What then was the result of this royal strife? King: The goddess of Argos transformed the woman into a cow. Chorus: And while she was a horned cow, did not Zeus approach her? King: So they say, making his form that of a bull lusting for a mate. Chorus: What answer then did Zeus' stubborn consort give? King: She placed the all-seeing one (panoptês) to stand watch over the cow. Chorus: What manner of all-seeing herdsman with a single duty do you mean? King: Argos, a son of Ge (Earth), whom Hermes slew." - Aeschylus, Suppliant Women 292
"The son of Ekbasos [son of Argos, son of Zeus and Niobe] was Agenor, and his son was Argos surnamed Panoptes. This man had eyes all over his body. He was extremely powerful, and killed the bull that raged through Arkadia whose hide he used as a cloak. He also ambushed and slew and Satyros who was hurting the Arkadians by stealing their herds. It is also said that he waited until Ekhidna fell asleep, and then killed her. She was a daughter of Tartaros and Ge, who would kidnap travellers passing by. Argos also avenged the murder of Apis by slaying the guilty pair, Thelxion and Telkhis … Zeus seduced Io while she was a priestess of Hera. When Hera discovered them, Zeus touched the girl, changed her into a white cow, and swore that he hand not had sex with her … Hera demanded the cow from Zeus, and assigned Argos Panoptes as its guard. Pherekydes says that this one was Arestor’s son, but Asklepiades says he was a descendant of Inakhos. Kerkops calls him a son of Argos and Asopos’ daughter Ismene [and so an uncle of Io], while Akousilaus says he was Earth-Born.) Argos tied the cow to an olive tree in the grove of the Mykenaians. Zeus instructed Hermes to steal her, and Hermes, unable to sneak her out because Hierax had told on him, killed Argos with a stone. From this came Hermes’ surname Argeiphontes." - Apollodorus, The Library 2.4