How are Poseidon and Neptune similar?
They are the same person but the Greeks called him posiedon and the Romans called him Neptune.
Poseidon and Neptune are the same person. Poseidon is the Greek terminology, and Neptune is the counterpart. But how strong is Neptune/Poseidon? He's the god of the seas and his main domain is the ocean. Just think of how much damage water alone can cause - earthquakes, hurricanes, typhoons. He's pretty powerful.