No the gods like Zeus Poseidon does not exist in the old day because he is basically a mythological god.
Poseidon really didn't get killed, Gods never die, or at least, immortals don't. They just keep coming back. Poseidon doesn't exist today because it's a myth.
The Trident of Poseidon
The Greek gods go very far back. We do not know exactly how far.
poseidon
No, Zeus is. Poseidon is god of the sea
Poseidon's relatives are all of the 12 major gods
Poseidon was one of the gods of the ancient Greek religion.
Poseidon's relatives are all of the 12 major gods
Cronus was a titan and the father of the majority of the Greek gods including Zeus, Poseidon, and Hades.
Poseidon is a male and therefore a god. Other examples of greek gods are Zeus, Hades, and Apollo. Female "gods" are called goddesses.
Poseidon is the brother of Zeus. Zeus is the god of the sky and the king of all gods. Poseidon is the god of the water and earthquakes, he is a very powerful god. He and eleven other gods are the twelve main gods even though there are many other gods.
Poseidon did not "become" God of the sea. The Greeks essentially 'created' the Gods to understand their world. ex. Poseidon created the sea storms, Helios carried the sun up into the sky every morning, and carried it back down every night.