These fall into several different categories. First are those whose origins are more or less lost in history – like Unicorns and Gorgons. There are those born out of popular Myth, like the Mermaids sailors "saw", like Giants, and also like Werewolves and Vampires. Then there are the creatures reported in Travellers' Tales like those of Sir John de Mandeville, who recorded various bizarre Monsters; in this context, some real creatures were for a while regarded as mythical – for a long time Europe refused to believe in explorers' reports of the rhinoceros.
see also: Animals Unknown to Science.