History of animals 2,13
Ἔστι δὲ καὶ τὰ μὲν κερατοφόρα τῶν ζῴων τὰ δ' ἄκερα. Τὰ μὲν οὖν πλεῖστα τῶν ἐχόντων κέρατα διχαλὰ κατὰ φύσιν ἐστίν, οἷον βοῦς καὶ ἔλαφος καὶ αἴξ· μώνυχον δὲ καὶ δίκερων οὐδὲν ὦπται. Μονοκέρατα δὲ καὶ μώνυχα ὀλίγα, οἷον ὁ Ἰνδικὸς ὄνος. Μονόκερων δὲ καὶ διχαλὸν ὄρυξ.
Καὶ ἀστράγαλον δ' ὁ Ἰνδικὸς ὄνος ἔχει τῶν μωνύχων μόνον·
English translation (D. W. Thompson, 1910)
Furthermore, of animals some are horned, and some are not so. The great majority of the horned animals are cloven-footed, as the ox, the stag, the goat; and a solid-hooved animal with a pair of horns has never yet been met with. But a few animals are known to be singled-horned and single-hooved, as the Indian ass; and one, to wit the oryx, is single horned and cloven-hooved.
Of all solid-hooved animals the Indian ass alone has an astragalus or huckle-bone;