In Greek mythology, Belus was a legendary king of Egypt, son of Poseidon and Libya (or Zeus and Libya), twin brother of Agenor in the common stemma, and father of the twins Danaus and Aegyptus. Genealogies also attach Cepheus, Phineus, and other children in minority traditions. Distinct from Babylonian Bel/Marduk and from other mythic figures named Belus.
Adapted from Wikipedia, "Belus (Egyptian)" · CC BY-SA 4.0
I.Twin of Agenor, father of twins
Apollodorus makes Belus and Agenor twin sons of Poseidon and Libya: Belus stays to rule Egypt while Agenor holds Sidon and Tyre. By Achiroe (Anchinoe), a daughter of the Nile, Belus fathers Danaus and Aegyptus; after his death their rivalry and the Danaïd marriages dominate the Argive foundation myth. Euripides, Apollodorus notes, added Cepheus and Phineus as further sons.
Hesiod's Catalogue gives him a daughter Thronia, mother of Arabus by Hermes. Other lists name Damno or Antiope as a daughter married to Agenor, or make Belus himself father of Phoenix and Agenor. Nonnus can call him "the Libyan Zeus" and connect him with the oracle of Zeus Ammon.
II.Later associations
Diodorus has Belus found a colony on the Euphrates and appoint Chaldean priest-astrologers on an Egyptian model. Pausanias credits him with a temple of Heracles at Babylon and reports a folk etymology deriving Latin bellum from his name as inventor of the sword. Learned writers also equate or confuse him with Bel-Marduk or Canaanite Baal—associations that sit beside, rather than replace, the Poseidon–Libya genealogy.
The atlas id belus is this Egyptian king, not the astronomical or Near Eastern god under the same Greek label.