Passage 8.25.5
πλανωμένῃ γὰρ τῇ Δήμητρι, ἡνίκα τὴν παῖδα ἐζήτει, λέγουσιν ἕπεσθαί οἱ τὸν Ποσειδῶνα ἐπιθυμοῦντα αὐτῇ μιχθῆναι, καὶ τὴν μὲν ἐς ἵππον μεταβαλοῦσαν ὁμοῦ ταῖς ἵπποις νέμεσθαι ταῖς Ὀγκίου, Ποσειδῶν δὲ συνίησεν ἀπατώμενος καὶ συγγίνεται τῇ Δήμητρι ἄρσενι ἵππῳ καὶ αὐτὸς εἰκασθείς.
For when Demeter was wandering while searching for her daughter, they say that Poseidon followed after her, desiring to unite with her. She changed herself into a mare and grazed together with the horses of Onkios; but Poseidon, realizing her deception, assumed the form of a stallion and thus united with Demeter.