Do not forget unicorns, vampires, griffins, zombies, werewolves, dragons and giants! I believe in mythology - tales and legends of ancient times. Dwarfism is a real condition, however... so they aren't legendary. Dwarves, like in the Lord of the Rings, are considered "fantasy" creatures rather than legendary.
In regards to mermaids, dragons and unicorns - there are too many stories from different parts of the world that are incredibly similar, especially when there was no contact between the different civilizations. Granted, it is possible that everything was just a trick of the mind or a trick of the eye - but for the Native Americans and the tribes of Africa to believe in the same thing before cultural interaction? That is a bit fishy to me.