Passage 8.25.8
ἐπάγονται δὲ ἐξ Ἰλιάδος ἔπη καὶ ἐκ Θηβαΐδος μαρτύριά σφισιν εἶναι τῷ λόγῳ, ἐν μὲν Ἰλιάδι ἐς αὐτὸν Ἀρείονα πεποιῆσθαι, οὐδʼ εἴ κεν μετόπισθεν Ἀρείονα δῖον ἐλαύνοι, Ἀδρήστου ταχὺν ἵππον, ὃς ἐκ θεόφιν γένος ἦεν· Hom. Il. 23.346 ἐν δὲ τῇ Θηβαΐδι ὡς Ἄδραστος ἔφευγεν ἐκ Θηβῶν εἵματα λυγρὰ φέρων σὺν Ἀρείονι κυανοχαίτῃ. Thebaid, unknown location. αἰνίσσεσθαι οὖν ἐθέλουσι τὰ ἔπη Ποσειδῶνα Ἀρείονι εἶναι πατέρα, Ἀντίμαχος δὲ παῖδα εἶναι Γῆς φησιν·
They bring in quotations from the Iliad as well as from the Thebaid as evidence to support their account. In the Iliad, Arion himself is mentioned: "Nor even if he drove behind noble Arion, Adrastus' swift horse, that was of divine descent." (Hom. Il. 23.346) And in the Thebaid, how Adrastus fled from Thebes: "Wearing mournful garments, together with dark-maned Arion." (Thebaid, unknown location) Thus, by these verses they wish to intimate that Poseidon was Arion's father, although Antimachus asserts that Arion was the offspring of Earth.