Was there any difference between their gods and mythology? Did Romans have those gods before they met the Greeks, or did they just take Greek gods and rename them?
https://www.reddit.com/r/AskHistorians/comments/63yfkg/why_does_roman_mythology_use_such_radically/
/u/a_wise_old_black_man wrote a comment that answers your 'why' question
Edit: the other comment in the thread answers the whether they were existing gods
https://www.reddit.com/r/AskHistorians/comments/6liefz/how_did_roman_religion_differ_from_greek/
I happened to find this great answer while looking around to answer another question, give it a read