Passage 6.20.15
παρεχομένου δὲ τοῦ ἱπποδρόμου παρήκουσαν ἐς πλέον τὴν ἑτέραν τῶν πλευρῶν, ἔστιν ἐπὶ τῆς μείζονος πλευρᾶς, οὔσης χώματος, κατὰ τὴν διέξοδον τὴν διὰ τοῦ χώματος τὸ τῶν ἵππων δεῖμα ὁ Ταράξιππος. σχῆμα μὲν βωμοῦ περιφεροῦς ἐστι, παραθέοντας δὲ κατὰ τοῦτο τοὺς ἵππους φόβος τε αὐτίκα ἰσχυρὸς ἀπʼ οὐδεμιᾶς προφάσεως φανερᾶς καὶ ἀπὸ τοῦ φόβου λαμβάνει ταραχή, τά τε δὴ ἅρματα καταγνύουσιν ὡς ἐπίπαν καὶ οἱ ἡνίοχοι τιτρώσκονται· καὶ τοῦδε ἡνίοχοι ἕνεκα θυσίας θύουσι καὶ γενέσθαι σφίσιν ἵλεων εὔχονται τὸν Ταράξιππον.
When the hippodrome stretches forward, passing along one side to a greater length, there is on the larger side, which consists of an embankment, at the point where the passage runs through this embankment, an altar-like shape known as Taraxippus ("Horse-frightener"), which brings terror to horses. Its form is circular. As the horses dash past this spot, a sudden and powerful alarm seizes them without any evident cause, and from this fear comes confusion: chariots commonly crash, and the charioteers are injured. For this reason, the chariot-drivers offer sacrifices and pray that Taraxippus may be gracious to them.