No, legendary creatures, maybe, but not mythical ones. Legend is usually based on some sort of historical fact and tends to mention real people or events, albeit in exaggerated form. In contrast myth is a type of symbolic storytelling that was never based on fact. Through time myths have been used to try to explain difficult concepts,(i.e, the origin of the universe) with the help of stories and fictitious allegories. Reference to many of the mythical creatures can be attributed to the, alcohol induced, wild stories told by 'ancient mariners' returning from exotic parts of the world where weird marine creatures such as manatees exist. There are those who suggest that the myth of mermaids are based on this ugly sea dwelling mammal. I find it mildly irritating to note how the two words, Myth and Legend are regularly used interchangeably to refer to the fictitious nature of something when, historically and academically there is a difference.

No, there is a reason we call them mythical. If you were to say are the things mythical creatures were BASED of of, then yes of course. Mythical creatures are the product of someone seeing something ordinary and making it seem more extraordinary, usually by combining it with something else and giving it magical powers. For example: The phoenix is based off of birds (idk if its any one particular bird), unicorns are based off of horses (maybe narwhals too?), griffins are based on lions and eagles, etc.

So in conclusion, are mythical creatures based on fact? Yes, in the same way that any other form of fiction is. But are they themselves real? No, at least they can't be considered to be so until we have more proof than a few old stories.

If they were they wouldn't be called "mythical". Adam and Eve living with dinosaurs ..... That's "mythical".