Passage 6.20.18
ἀνὴρ δὲ Αἰγύπτιος Πέλοπα ἔφη παρὰ τοῦ Θηβαίου λαβόντα Ἀμφίονος κατορύξαι τι ἐνταῦθα, ἔνθα καλοῦσι τὸν Ταράξιππον, καὶ ὑπὸ τοῦ κατορωρυγμένου ταραχθῆναι μὲν τῷ Οἰνομάῳ τότε, ταράσσεσθαι δὲ καὶ ὕστερον τοῖς πᾶσι τὰς ἵππους· ἠξίου δὲ οὗτος ὁ Αἰγύπτιος εἶναι μὲν Ἀμφίονα, εἶναι δὲ καὶ τὸν Θρᾷκα Ὀρφέα μαγεῦσαι δεινόν, καὶ αὐτοῖς ἐπᾴδουσι θηρία τε ἀφικνεῖσθαι τῷ Ὀρφεῖ καὶ Ἀμφίονι ἐς τὰς τοῦ τείχους οἰκοδομίας τὰς πέτρας. ὁ δὲ πιθανώτατος ἐμοὶ δοκεῖν τῶν λόγων Ποσειδῶνος ἐπίκλησιν εἶναι τοῦ Ἱππίου φησίν.
An Egyptian man stated that Pelops, having received something from Amphion of Thebes, buried it at the place they call Taraxippus ("Horse-frightener"); and due to what had been buried there, Oenomaus' horses were thrown into confusion at that time, and afterwards horses have continued to be similarly troubled. This same Egyptian claimed that Amphion and the Thracian Orpheus were both great sorcerers, and as they sang incantations, animals came willingly to Orpheus, and stones moved themselves to Amphion during the building of his wall. But the most plausible explanation, in my opinion, is that Taraxippus is a title given to Poseidon Hippios ("Poseidon of horses").