After a long series of wars with Macedonia, the Roman army conquered the Greek peninsula, and as a consequence, Hellenic culture -- art, literature, and philosophy -- infiltrated much of Roman life. This infiltration included Roman religion. While their names did not change, the Roman gods began to become more closely associated with their Greek counterparts: Venus became more like Aphrodite, Pluto became Hades, Neptune became Poseidon, and lastly, Jupiter became Zeus.
The myths and legends of Roman mythology gradually grew distorted, and many Roman gods began to lose their identity and their individuality. However, one soon forgets that the Roman gods existed long before the arrival of the Greeks, before they lost their uniqueness.