1. A son of Apollo and Asteria, the daughter of Coronus (Schol. ad Apollon. Rhod. i. 139), or, according to others, of Apollo, by Antianeira, or Ampycus, or of Apollo and Cyrene. (Orph. Arg. 185, &c., 721; Apollon. Rhod. i. 139, &c.; Hygin. Fab. 14; comp. Val. Flacc. i. 228.) He was one of the soothsayers who accompanied the Argonauts : his name signifies "the knowing," and has been considered to be a mere epithet of Thestor or Mopsus. (Schol. ad Apollon. Rhod. i. 139.) He joined the expedition of the Argonauts, although he knew beforehand that death awaited him. He was killed in the country of the Mariandynians by a boar or a serpent; or, according to others, he died of a disease. (Apollod. i. 9. § 23; Apollon. Rhod. i. 140, 443, ii. 815, &c.; Val. Flacc. v. 2, &c.) The Megarians and Boeotians who were to found Heracleia, were commanded by Apollo to build the town round the tomb of the hero, and to worship him as the protector of the place. (Apollon. Rhod. ii. 846, &c.)

2. There are three other mythical personages of the name of Idmon. (Apollod. ii. 1. § 5; Ov. Met. vi. 8, 138; Stat. Theb. iii. 389.)