Zonus

Since Pliny is quoting Eratosthenes, presumably borrowed from Ancient Greek. W. W. Tarn understands the term as a separate, native name of the Oxus, rather than a corruption of Ὦξος (Ôxos) itself, in which case it would ultimately be from a Central Asian language.

• (Classical Latin) IPA^(key): [ˈzɔ.nʊs]

Zonus m sg (genitive Zonī); second declension

1. (hapax legomenon) A river flowing into the Caspian Sea, mentioned by Pliny; perhaps an alternative name for the Oxus (modern Amu Darya)
   • c. 77 CE – 79 CE, Pliny the Elder, Naturalis Historia 6.15.36:

     inde per Anariacos, Amardos, Hyrcanos ad ostium Zoni fluminis IIII DCCC,

     thence, through the territories of the Anariaci, the Amardi, and the Hyrcani, to the mouth of the river Zonus he makes four thousand eight hundred stadia,

Second-declension noun, singular only.

• Zonus in Gaffiot, Félix (1934) Dictionnaire illustré latin-français, Hachette.
• Tarn, W. W. (1948) Alexander the Great, volumes 2, Sources and Studies, Cambridge University Press, page 13, note 1