Seriously, you're asking if a book about vampires and werewolves is fiction or not? No, it's definitely fiction.
Non-fiction is a book with factual information. Some books, such as comic books or poetry are classified as non-fiction regardless.
Unless you've seen any vampires stalking around lately, no.
Mermaids are a fiction topic. They were first introduced when Greek fishermen caught sight of dugongs, but mistook them for young maidens with the top half being human features, but the bottom half...