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People are only ever human beings, never a different being even after death. A dog does not become a cat after death nor a pig become anything else after death.
We know this from Revelation 20 that the dead are to be born again by the breath of God to the form they had before but with immortality or to judgement and destruction by fire.
People are deceived by evil beings into believing that after death they may transform into another kind of creature. This transformation concept comes from Eastern mysticism.
We also know that Satan can change his appearance to look like an angel of light so it stands to reason that his minions may do the same and appear in the form of dead people to deceive the uneducated. These are demons/evil/unclean spirits spleen of in scripture.