Poseidon never died in Greek Mythology, being a god both immortal and ageless he can not die.
If you believe in Greek mythology, yeah.
In Greek mythology when Athena is the daughter of Zeus, Poseidon being the brother of Zeus, is her uncle. However, myths exist which make Poseidon the grandfather of Athena when she is the daughter of his son Triton, or Poseidon is the father of Athena when her mother is Tritonis or Polyphe.
Minotaurs do not exist, it was a creature from Greek mythology
In Greek Mythology, yes.
No, it is a creature in Greek mythology
No one; the individual does not exist in Greek myth.
Vampires don't exist... they are Mythology.
No.
Janus did not exist in ancient greek mythology.
Greek Mythology pre-dates Noah and the tales of the Bible; so, yes, it did, so too did Egyptian Mythology.
Poseidon
Yes, they are a part of Greek beliefs, and are in Greek stories, but that does not mean they do not exist.