Why do Roman names end with ''us''?

Answer 1

No, there are other ending for Latin names. The masculine names often end in 'us' and their feminine counterparts end in 'a', but there are endings such as Cato, Caesar, Agrippa, and Vindex, which do not have the 'us' ending.

Answer 2

All of the Roman names do not end in us, but a great many of them do. The "us" is the masculine singular in Latin, the language of the Romans. The feminine form of a Latin name drops the "us" and replaces it with an "a". For example, Julius and Julia. However there are other Latin names that end in "o" such as Piso or Pollio, and other Latin names that end in "a" even though they are masculine, such as Sulla and Dolobella.