Dragons do exist... in the form of a Dinosaur called Dracorex Hogwartsia, which- though was a relative of the cephaloids looked much like a dragon (though, if you watch(ed) the T.V programme 'Primeval' Dracorex doesn't have any evidence of 'wing like' frills along the back.)
Here's the Skull of Dracorex Hogwartsia;
And here is what Paleo artists think the creature looked like:
They we not carnivorous, but herbivorous and used the bony lumps on its skull to literally 'lock-heads'.
Dragons as a fire-breathing serpentine creature is
very unrealistic- granted, there are lizards today that glide using rib muscles and skin flaps, but they aren’t very big. I have just done a course on anatomy, very hard stuff. And- like a bird you need flight muscles to flap up and down- ever wondered when you was a kid and you saw cartoon characters flap their arms and fly, and wondered why you couldn’t do it yourself (even with wing-like structures on your arms). Birds have a breast Bone (if you’ve eaten chicken its that big white bone that has all the dry chicken meat on it) this connects muscles that enable powerful ‘up and down’ strokes. That big bone- just for a few movements of the wings. Pterosaurs (prehistoric flying reptiles) used muscles in their arm and chest to propel their flight, but the larger pterosaurs mainly glided. This anatomy also applies to ‘Angels’ (an possibly demons if you envision demons as winged creatures) like I said before- you need very big and very strong muscles on your chest to propel flight, and, if Angel existed, they would look more avian.
Another mythical creature that 'existed' was the gryphon, the Persian/Mongolian Half-lion, half-eagle creature that guarded its eggs and gold with all its might. These creatures existed... as Protoceratops- a late-creations dinosaur with a beak- it was small and a relative of the famous ceratopceian: Triceratops.
Q: but how on earth does a reptilian-like dinosaur end up with wings and feathers?
A: Protoceratops had a 'beak' and teeth. And its fossils had been found huddled around fossilised nests still containing fossilised eggs. As a ceratopceian, it had a frill at the back of its head- possibly to shield its neck and attract mates. sometimes, fossils of dinosaurs are broken up- actually 99.9% of the time, whole fossils are rare to come by, with that said- it's possible that some of the frill of the Protoceratops broke away from its skull and landed by its shoulders- making it look like a wing.
Protoceratops skeleton:
it has also been suggested that Protoceratops may of had some sort of feathering along its tail- that may of been preserved to create the classic 'lion tail'.
As for Unicorns, they may have been misidentified stags, I’ve seen some stags that have lost a horn. Or, the ‘horn’ could be from male Narwhals, as their tusk is long and spiral in shape- these creatures, much like elephants could have been killed for their tusk to be sold on a unicorn horns:
I was recently studying the Lock-ness Monster and the lake. And you would think that it is full of lots of big yummy fish to eat. It’s not, not enough to sustain a full grown plesiosaur at least- the lock is rather cold too- never freezing, but still cold, and plesiosaurs are described as marine
reptiles and, having a reptile of my own- when they get cold they become very sluggish- so, if ‘Nessie was in the Lock Ness- it would of died of drowning ages ago due to it being a marine reptile. Though, Being British myself- I do know that Britain was home to a vast array of sea reptiles in the Jurassic(?) and it is possible that someone (in Scotland) dug up or saw a plesiosaur skull/skeleton and thought that they must have been living in the lock. As for most of the sightings of ‘Nessie. They are caused by floating Logs, bird and even otters.
Mythical sea creatures may have a grain of truth to them- at least in my eyes (over the years of disappointment and fakes, I’ve grown sceptical of seeing ‘real’ mythological sea creatures)
Mermaids: They are an imposibilially, at least in the sense of half-fish, half-human concept, but sailors could have mistaken manatees and seals for mermaids.
Q/Statement: but mermaids are beautiful women! How can sailors mistake manatees or seals for them?
A: to survive in the water- especially for a mammal, you need a layer of blubber to insulate yourself (even in tropical oceans), with the salt water you would get wrinkling of the skin and mermaids wouldn’t be able to keep the hair on their heads (forgot the reason, I think it’s to do with the salt water), making a mermaid look rather like a manatee- or a very old, rather large mermaid.
There is, however a condition simply known as ‘mermaidism’ where the legs are fused together in foetal development- these children don’t usually live very long (of what I have heard)
Krakens:
Now, these creatures I DO believe in, maybe because they’ve been discovered under the name of ‘colossal Squid’ which can grow up to 35 ft. long, but oceanographers claim to have seen one as big as 132ft! Sperm Whales fed on these squid- that’s why sometimes, when a beached sperm-whale is on the beach, you can see ring-like scars where the squid have grasped at the whale, when the whale has them in its jaws.
:WARNING: some people may find this image disturbing
But most of these creatures are stories, told on from generation to generation. Maybe, like today, some people fooled others into believing creatures existed, or maybe they told tall tales, much like the tale ‘of the one that got away’ exaggerating the event.