There is a long, long history of equating the Greek and Roman deities. And it’s not without reason–the Romans did, in fact, take great parts of Greek myth and deity attribute and apply them to their own gods. Many if not most people think of the two pantheons as the same gods under different names. But the Roman gods existed, and were honored, before this. While it is often difficult to work out just where and how the conflation occurred, and why the two pantheons are so similar in the first place, it’s an interesting and worthwhile study.
Short answer: The Roman gods were and are the Roman gods. The Romans did not simply take the Greek gods for their own and rename them. But the Romans had something called the interpretatio romana in which they identified foreign gods with their own deities. I’ve mentioned this elsewhere with regard to the Celtic gods of Gaul, but it’s something they tended to do regardless of where they went. And just as the British goddess Sulis became identified with the Roman Minerva, so did the Greek goddess Athena. Thus, many of Athena’s stories became associated with Minerva as well. (Which is not to say that those tales do not now belong to Minerva, too.)
Slightly longer answer: Rome, like Greece, came from older Indo-European origins. They (and their gods) have common ancestors. So it is also that rather than Jupiter deriving from Zeus, both gods derived from a more ancient ancestor, the reconstructed Proto-Indo-European *Dyeus Pater or “sky father,” and are cousins to the Norse Tyr. Since the Romans did, much much later, borrow from the Greeks as well, it’s now hard to say how much myth and deity attribute were common to both before that occurred. Minerva and Athena share a common ancestor, but it’s safe to say that Minerva became more “Athena-like” after the Romans encountered the Greeks.
(I’m adding a parenthetical caveat to say that I am not here discussing the differences between Greek and Roman religious practices, which were considerable and which certainly affected and affect relationship with deity. That’s a topic for a much longer day. I am also not considering the Etruscan gods in this post–again, another topic, another day.)
In some cases the Greek and Roman deities seem fairly similar–Zeus/Jupiter and Hera/Juno, for example, although in both cases the Roman gods do simply seem more Roman; Juno, to me (UPG warning), seems more matronly and perhaps dignified than the common perception of Hera.
In other cases there were noticeable differences. The Roman Vesta maintained a larger public presence than did the Greek Hestia. The Roman Venus was originally associated with gardens, fruit and flowers; while she also shares much with the Greek Aphrodite, to me she “feels” rather earthier than Aphrodite and lacks some of Aphrodite’s connection with the sea (again, UPG warning). And the Roman Mars retained his agricultural connections along with his warlike attributes and remained a widely popular deity, while the Greek Ares was less beloved.
And of course there are the many gods who simply have no Greek equivalent–Janus comes first to mind but there are numerous others.
Further reading:
Beard, Mary, John North and Simon Price. Religions of Rome: Volume 1, A History. Cambridge University Press: Cambridge, 1998.
Lipka, Michael. Roman Gods: A Conceptual Approach (Religions in the Graeco-Roman World). Brill: Boston, 2009
Scheid, John. An Introduction to Roman Religion. Trans. Janet Lloyd. Indiana University Press: Bloomington, Indiana, 2003.