Passage 8.25.10
δύναιτο δʼ ἂν καὶ ἀναφύντι ἐκ γῆς τῷ ἵππῳ ἐκ θεοῦ τε εἶναι τὸ γένος καὶ αἱ τρίχες οἱ τὴν χρόαν ἐοικέναι κυανῷ. λέγεται δὲ καὶ τοιάδε, Ἡρακλέα πολεμοῦντα Ἠλείοις αἰτῆσαι παρʼ Ὄγκου τὸν ἵππον καὶ ἑλεῖν τὴν Ἦλιν ἐπὶ τῷ Ἀρείονι ὀχούμενον ἐς τὰς μάχας, δοθῆναι δὲ ὑπὸ Ἡρακλέους ὕστερον Ἀδράστῳ τὸν ἵππον. ἐπὶ τούτῳ δὲ ἐς τὸν Ἀρείονα ἐποίησεν Ἀντίμαχος ὅς ῥά ποτʼ Ἀδρήστῳ τριτάτῳ δέδμηθʼ ὑπʼ ἄνακτι. Antimachus, unknown location.
One could believe also from the horse's having sprung forth from the earth that its lineage was divine, and also from its hairs, whose color resembled dark blue. This story too is told: that when Heracles was waging war against the Eleans, he requested the horse from Oncus, and having received Arion mounted him into battle and so captured Elis; and that afterward, Heracles gave this horse to Adrastus. It is about this very Arion that Antimachus composed the following lines: "Who once was mastered by lord Adrastus as the third possessor."