Robust Redhorse Sucker fish
What do these things have in common? They were all thought to be extinct until someone found them to be living.
All the time it seems we are discovering species either never heard of or once thought to be extinct. Until the gorilla was discovered in Central Africa in 1847, no one believed anything like that existed. The Coelacanth was long thought to be extinct until one was found alive in Madagascar 1938. The Robust Redhorse Sucker Fish was thought to be extinct because no one had seen one for years, then in 1991 a man caught five of them right outside of Atlanta.
More? Baird's tapir was discovered in Central America in 1863. The giant panda was found in China in 1869, but not collected alive until 1936. In Mongolia in 1881 Przewalski's horse was discovered. The mountain gorilla, a subspecies, was found in East Africa in 1902. The pygmy chimpanzee, was described in 1929, but not brought back to Europe from Zaire until the late 1930's. The Chacoan peccary, a Pleistocene fossil form, was discovered alive in Paraguay in 1975. The Megamouth, a 15-foot shark, representing a completely new species, genus, and family, was discovered in Hawaii in 1976.
Creatures are turning up all of the time and scientists even have a name for them, living fossils. So just because we haven't seen a mermaid, just because there is no fossil evidence, doens't mean that there isnt' a chance that perhaps they do exist.
And even if they don't, isn't it wonderful to imagine that they do?