Mermaid/werewolves/vampires.. do they exist? not the way they r in movies but in some other ways.. for example if u can breath in water and swim fast.. or suck something else thats not blood or so.. do these things exist?
Typical vampires, werewolves, and vampires do not exist. It is not possible for someone to change their DNA to become any of those things.
As for the 'other things' you asked about:
Psi vamps drain people of energy instead of blood and do not kill their victims.
There are medical conditions that can make people think that they are werewolves, but will not turn them into one.
For mermaids, I have no knowledge of anything similar that is real.