That of several figures in Greek mythology, e.g.:
(1) I. son of Naubolos; one of the Argonauts (and cf. Homer, Iliad 2.518).
(2) I. son of Eurytos, the king of Oichalia. When Heracles was accused by Eurytus of having stolen his mares, Iphitus was put in charge of searching for them; but later Heracles, in a fit of rage, killed him by hurling him from the walls of Tiryns. See e.g. Homer, Odyssey 21.14ff; Apollonius Rhodius, Argonautica 1.86 & 207; Apollodorus, Library 2.6.1-2.
(3) I. the king of Elis, known for the contribution he made (following an oracle's command, as the legend says) to the (re-)organization of the Olympic Games in (trad.) 776 BCE. See e.g. Pausanias 5.4.5-6 and passim.