Passage 7.21.8
ὠνομάσθαι δὲ Ἵππιον τὸν θεὸν πείθοιτο μὲν ἄν τις καὶ ἐπʼ αἰτίαις ἄλλαις· ἐγὼ δὲ εὑρετὴν ἱππικῆς ὄντα ἀπὸ τούτου σχεῖν καὶ τὸ ὄνομα εἰκάζω. Ὅμηρος μέν γε ἐν ἵππων ἄθλοις Μενελάῳ κατὰ τοῦ θεοῦ τούτου πρόκλησιν περιέθηκεν ὅρκου· ἵππων ἁψάμενος, γαιήοχον ἐννοσίγαιον ὄμνυθι μηδὲν ἑκὼν τὸ ἐμὸν δόλῳ ἅρμα πεδῆσαι. Hom. Il. 23.584-585
Certainly one might be persuaded that the god was named Hippios ("Horse-god") for other reasons as well; but in my own view, since he was the inventor of horsemanship, he probably derived his name from this circumstance. Indeed, Homer, in the contest of horses, makes Menelaus swear an oath with an invocation of this very god: "Lay hands upon the horses and swear by the Earth-shaking Earth-holder that you did not willingly impede my chariot through guile." (Homer, Iliad 23.584-585)