-eos does exist in old Latin (Old, Classical or Late Latin), but -ios seems to be New Latin or at best Medieval Latin.
The only term in polis where I saw an explicitaly given reference for a Greek genitive by a dictionary was Pentapolis with Pentapoleos in "Ruf. Fest. brev. 13". Rufius Festus does indeed have that genitive (in some printed editions at least, this has "Pentapolis"). In old Latin one can find other terms with gentive -eos, and at least in Late Latin also for terms in polis (for example Hieronymus Stridonensis has "Neapoleos"). In New Latin one can find terms with gentive -ios and also for some terms in polis. Even some grammars published during the 16 till 19th century do mention a genitive -ios (for example John Milton in the 17th century has "Neapolis, neapolios, acc. neapolin" - which BTW might be more common in English New Latin as the English sometimes (mis)pronounce e as i). But is there any term with genitive -ios in old Latin? -84.161.25.79 16:21, 24 April 2017 (UTC)