On earth there were are certain animal species before which are now extinct but on the other hand there are also existing new ones. (Every year there are discovered new ones). This has often to do with the natural capicity of the organism to adapt itself to the ever changing environment. We normally would call this "evolution", but this word is however nowadays understood like: "the grow to a higher grade of development", although it is actually about adaptation and not growth.
In certain ancient texts there are described creatures which we don't know of today and because we can not find any physical evidence for these it is often thought that therefore they must be made up. While some creatures - including dragons and serpents - were sometimes used for merely symbollic reasons, they may once have existed for real. Besides the old mythologies from around the world we also find these kind of creatures in the old Jewish and Hebrew texts. It is possible that the Jewish legends are the oldest writings known so far about creatures which nowadays don't exist (anymore), like the phoenix; a bird which at the end of her life cycle lighted her own nest including herself into flames, from where a new baby bird came from the ashes where her soul could enter back in for another 500 years.
It is little known that the wild horned version of the horse; the "unicorn", is also mentioned in several books of the Bible: See Job 39:9-10, Numbers 23:22 and 24:8, Deuteronomy 33:17, Psalm 22:21, 29:6, 92:10 and Isaiah 34:7. In fact, unicorns were still depicted on 15th and 16th century European tapestries and renaissance man Leonardo da Vinci wrote about the unicorn in one of his notebooks, making it plausible that this animal wasn't a fantasy animal but did probably exist in the past. (See Wikipedia for more information.)
Dependant on the Bible translation, in the Biblical book Isaiah the "basilisk" and the "fiery flying serpent (or dragon)" had been mentioned in a literary sense as if they really would have existed in the past. (See these different translations.) From Isaiah 30:6 (King James Version):
"The burden against the beasts of the South. Through a land of trouble and anguish, from which came the lioness and the lion, the viper and the fiery flying serpent, they will carry their riches on the backs of young donkeys, and their treasures on the humps of camels, to a people who shall not profit;"
These flying serpents are perhaps related to the so-called "lindworms" from European mythology and folklore, where they were described here as serpent-like half-dragons that could be either winged or wingless, plus quadrupedal, bipedal or limbless and with a poisonous bite. From stories like the "Lambton Worm" it is evident that the word "worm" was an old word for a serpent-like and dragon-like creature, essentially a kind of sea serpent but that also could live on dry land.
The image above-left is
a piece from the walls of the Babylonian "Ishtar Gate". The walls of
the Ishtar Gate have been reconstructed to a hight of 14 km and are now
currently in the Pergamon Museum in Berlin. Here on the walls, among
the known depictions of numerous creatures such
as: lions, bulls, horses and the now extinct aurochs, there are also
numerous depictions of a creature that is known today as the
"mushhushshu", which can be loosely translated as "splendor
serpent" in Akkadian. There is no doubt that this actually is a mixed
creature, composed of parts from multiple animals. Depictions of this
creature had been
mostly unchanged in ancient Babylonian art for centuries, which did
German archeologist and architect Robert Koldewey (the discoverer of
the Ishtar Gate between 1899 and 1914) to the belief that this
creature really could have existed in the past. The image above-right
probably much older depiction from a cylinder seal.
This creature is actually very similar the so-called "questing beast" ("questing" as the old word for "barking") or "beast glatisant" (French) from the King Arthur legends, in which it is described having the head of a serpent, the body of a leopard, the backside of a lion and the front paws of a reindeer or a rabbit. From his belly would have sounded a sound like thirty yelping dogs. Popular interpretations view the mushhushshu as a dragon or dragon-like creature.
In the scriptures including the Bible, there is also mention of beings that are called "cherubim"
(singular: "cherub") which were seen as servants of the "Lord", and also were mixed creatures . The
term "cherubim" is cognate with the Assyrian "karabu" (meaning "great",
"mighty"), and the Akkadian and Babylonian term "kuribu" (meaning:
"propitious", "blessed".) Among the Karibu there is the "Lammasu" (or "Sedu); a
human-headed lion (like the Egyptian sphinx) with eagle's wings and the human-headed
winged bulls. Those could also be symbolical references to the
astrological age of Leo (the lion) and the age of Taurus (the bull), which began in 4,300 BC
according to Neil Mann's interpretation, which actually is within
the timeframe when the Mesopotamian culture flourished.
Ancient Arabian legends tell of the nations that were created before
the time of Adam (the first human) that were called "Djinn" (singular) or "Djinni"
(plural). (This is where the word "Genie" comes from). The
Djinni were humanoid beings whereof many
would have features that we would call "animalistic" today, like having the head of a bird or a dog. From the text: "Akhbar Al Zaman" (The History of Time),
translated by Jason Colavito based on the French edition of "Baron
Carra de Vaux", published in 1898 as "L’abrégé des merveilles":
"There is a race where the individuals are tall and very agile and have wings, and whose language is formed by the snapping of fingers. In another race, the individuals have the bodies of lions and birds’ heads, covered in hair and having long tails, and their language is a buzzing. In another, they have two faces, one in front and one behind, and several feet; their language is similar to that of birds. These nations are the djinn; there is among others a kind of djinn which has the form of dogs, complete with tails; their language is an incomprehensible growl. In another of these races, individuals resemble men except that they have their mouths in the middle of their chests and speak by whistling. Another race is similar to long snakes provided with wings, legs, and tails; others are like halves of men, having only one eye, a hand, and a foot and walking by jumping and bounding; their language resembles that of cranes. Others have the faces of men and loins covered tortoiseshell like turtles; they have claws for hands, long horns on their heads, and their language is similar to the howling of wolves. Others have two heads with two faces like the heads of lions; they are great and speak an incomprehensible language. Others have a round face, white hair, tails like oxen, and they spit fire from their mouths. Others look like women, with hair and breasts; there are no males in this race; these women are made pregnant by the wind, and they bring forth that individuals who resemble them; they have lovely voices and they attract many people of other races by the charm of their voices. Others are shaped like reptiles and insects. Although they are tall, they eat and drink like cattle. Still others are like the beasts of the sea; but they have tusks like wild boars and long ears. The rest of these twenty-eight breeds are of various forms, and all have a wild appearance.
They say that these nations interbred, and that the number of distinct races grew to one hundred and twenty."
The Mesopotamian texts describe and depict seven sages who were known as Apkallu (Akkadian) or Abgal (Sumerian) and were said to have been created by the god Enki to establish culture and give civilization to mankind. They served as priests of Enki and as advisors or sages to the earliest kings of Sumer before the great flood. They would have given moral code (Me), crafts and arts to mankind. They are depicted as humanoids with wings and the head of a man or bird (see images above), and sometimes the lower body is depicted like the tail of a fish.