Romans knew them as Horatius
, yet we know them as Horace and Propertius. Were we consistent, we would know them both as either Horace and Properce or Horatius and Propertius. The reason for the odd spelling of Horace is that Horace was popular and so his name was standardized during English's French-influenced period, before we spoke Modern English; the much less well-known Propertius was standardized later, when a different convention was used. Since there are few names like Horace's this isn't much of a problem, but there is a far greater one when trying to figure out Greek names. That's mostly because the ancient Greek alphabet uses different letters from the Latin or English alphabets
Here is a table showing what termination you're likely to find on Greek proper names, nouns, and adjectives, and how those letters are transliterated into Latin.
"Transliteration or Transcription of Greek," by Gerald P. Verbrugghe. The Classical World, Vol. 92, No. 6 (Jul. - Aug., 1999), pp. 499-511.
|-η||-a or long e|
|-ων||-o or -on|
|-αιος||-aeus or -ensis|
|-ειων||-ensis or -ides|