Passage 7.21.9
Πάμφως δέ, ὃς Ἀθηναίοις τοὺς ἀρχαιοτάτους τῶν ὕμνων ἐποίησεν, εἶναί φησι τὸν Ποσειδῶνα ἵππων τε δωτῆρα νεῶν τʼ ἰθυκρηδέμνων. Pamphos, work unknown οὕτω διὰ τὴν ἱππικὴν καὶ οὐκ ἀπὸ ἑτέρας προφάσεως τὸ ὄνομα ἔσχηκεν.
Pamphos, who composed the most ancient hymns for the Athenians, says that Poseidon is the giver of horses and ships with upright hulls. Thus, it is from his association with horses, and not from another cause, that he has received this name.