Eberhard Werner Happel published in 1691 “Relationes Curisosae”, a collection of curiosities which reports the usual stories from exotic countries, scientific discoveries of various disciplines as well as descriptions of landscapes, their inhabitants and folk lore.
He states that they are monsters or miraculous creatures which could not spring from an ordinary copulation of two common animals of different species. Arguing that it is well accepted that dragons inhabit the remotest dwellings such as caves, cliffs or deserts, he noticed that eagles, vultures and other birds of prey are their companions.
In fact they dwelled there even before the dragon. To these places they brought their prey , snakes, birds, rabbits, lambs, dogs, and even little children, to lacerate and devour them. And there the remains decayed. But still remnants of the semen of these unlucky victims survived. Of course it was impossible that this seed could develop ordenary animals.
But through time the semen of various creatures would intermingle and at last a kind of "fermentational putrefaction" would give birth to a dragon. Logically, this dragon will show features of all animals involved: head and tail of the snake, wings of a bird or bat, ears of a rabbit and legs of whatever kind of being.