Sir Richard C. Jebb, Commentary on Sophocles: Trachiniae

φάσμα ταύρου, a periphrasis expressing his dread aspect: cp. O. C.1568“σῶμά τ̓” | “ἀνικάτου κυνός”: Verg. Aen.6. 289et forma tricorporis umbrae (Geryon).

Acheloüs fights, then, as the “ἐναργὴς ταῦρος”,—not merely as the “ἀνδρείῳ κύτει βούπρῳρος” (12). Sophocles is here following the traditional version. The Homeric Scamander, in conflict with Achilles, roars ‘like a bull’ (“μεμυκὼς ἠΰτε ταῦρος”, Il.21. 237). “ἐντεῦθεν ὁρμηθέντες” (says the schol. there) “τὸν Ἀχελῷον ἐταύρωσαν Ἡρακλεῖ ἀγωνιζόμενον”. The taurine form was given to Acheloüs, in that combat, by Archilochus (schol. ib.), by Pindar (schol. Il.21. 194), and by the logographer Pherecydes ( Apollod.2. 6. 5): perhaps, too, by Panyasis, the author of an epic “Ἡράκλεια”. An engraved gem in the British Museum (King, Antique Gems II. pl. 34, fig. 3) shows Acheloüs as a bull, preparing to butt at Heracles. The gem is older than the time of Sophocles, and may, as Mr. Murray S. thinks, have followed the rendering of this subject on the still more archaic throne of Apollo at Amyclae ( Paus.3. 18. 5). Cp. n. on 520. This fight was a favourite theme in art: for the literature, see Roscher, Lex.p. 9.

Ovid (Met. 9. 1—100) departs from the old Greek version: his Acheloüs begins the fight in quasi-human form,—then turns into a serpent (63),—and then, as a last resource into a bull (80).