Nebuchadnezzar, King of Babylon, the faithful prince appointed by the will of Marduk, the highest of princely princes, beloved of Nabu, of prudent counsel, who has learned to embrace wisdom, who fathomed their divine being and reveres their majesty, the untiring governor, who always takes to heart the care of the cult of Esagila and Ezida and is constantly concerned with the well-being of Babylon and Borsippa, the wise, the humble, the caretaker of Esagila and Ezida, the firstborn son of Nabopolassar, the King of Babylon.
Both gate entrances of Imgur-Ellil and Nemetti-Ellil following the filling of the street from Babylon had become increasingly lower. Therefore, I pulled down these gates and laid their foundations at the water table with asphalt and bricks and had them made of bricks with blue stone on which wonderful bulls and dragons were depicted. I covered their roofs by laying majestic cedars length-wise over them. I hung doors of cedar adorned with bronze at all the gate openings. I placed wild bulls and ferocious dragons in the gateways and thus adorned them with luxurious splendor so that people might gaze on them in wonder
I let the temple of Esiskursiskur (the highest festival house of Markduk, the Lord of the Gods a place of joy and celebration for the major and minor gods) be built firm like a mountain in the precinct of Babylon of asphalt and fired bricks.
While the Ishtar Gate is the source most often cited when referencing the mushhushshu there is at least one written account of the Babylonian 'dragon'. In the Apocrypha, a collection of stories excluded from the Protestant Bible, is found the Book of Bel and the Dragon. In the Book is a story of how Nebuchadnezzar kept a dragon in the temple of the god Bel and when the prophet Daniel denounced the worshipping of false gods the king introduced Daniel to the dragon saying it "liveth and eateth and drinketh; you cannot say that he is no living god; therefore worship him." Daniel feed the dragon lumps of tar, fat, and hair that killed the animal proving it was mortal and not a god.
Behold now behemoth, which I made with thee; he eateth grass as an ox. Lo now, his strength is in his loins, and his force is in the navel of his belly. He moveth his tail like a cedar: the sinews of his stones are wrapped together. His bones are as strong pieces of brass; his bones are like bars of iron. He is the chief of the ways of God: he that made him can make his sword to approach unto him. Surely the mountains bring him forth food, where all the beasts of the field play. He lieth under the shady trees, in the covert of the reed, and fens. The shady trees cover him with their shadow; the willows of the brook compass him about. Behold, he drinketh up a river, and hasteth not: he trusteth that he can draw up Jordan into his mouth. He taketh it with his eyes: his nose pierceth through snares.
The depiction on the gate shows a long necked quadruped with scales, clawed hind feet, lion-like front feet and legs, a forked tongue and a horn on its nose (head?). The Biblical description of the behemoth is more about it habits and habitat than its appearance but it does seem to indicate a large, powerful herbivore that prefers a riparian habitat, can swim and has a horn on its nose. The Book of Bel and the Dragon just shows it's not a picky eater. Adding that all together we have something weird in the woods down by the creek. There doesn't seem to be any known animal that fits the above combination of traits. The scales and forked tongue would indicate a reptile but the plumed tail (and it does appear to be a plume and not a stinger) and the grass eating would indicate a mammal. Throw in the clawed hind feet, especially the rear pointing claws, and it points toward a bird. A chimera, indeed.
But what happens if we remove one or more of the incongruous traits? But why would we do that? If the 'dragon' was a living creature it was depicted as the artist actually saw it, right? Maybe not. Let's assume that the 'dragon' was actually captured and brought to Babylon from deepest, darkest Africa or some other far away place but that it was a one-time event. One 'dragon', one time, seen by some of the people of Babylon. I say some, rather than all, since if the 'dragon' was placed in either the temple of Bel or in the Hanging Gardens both were places not open to the general public. Even if it were a long-lived creature, say on the order of an elephant, 50 years or so would probably be the outside limit to how long it could have lived in captivity. But if Daniel killed the 'dragon' it may have been alive there for a small handful of years. During its life it may have been seen by thousands of people but very few would have been artist or craftsmen. But for the few that were, probably only a small number would have found a reason to create an image of the 'dragon' in their work. So now we have a few craftsmen/artist who may, or may not, have actually seen the 'dragon, whose depictions, along with stories, may have been the inspiration for the other images created over the centuries. So both sides of the real/mythological argument could be right. It could have been a real animal that made a one-time appearance in Babylon then slipped into the mythos of the Babylonian gods.
Reagrdless of whether reptile or mammal the 'dragon' was real to the people of Babylon during the time of King Nebuchadnezzar. If the creature was part of a remnant population of some form of mega fauna that escaped the Pleistocene, or even earlier, extinction or a complete unknown that has evaded modern science, it was/is probably the ragged end of a line with a small population. And if it still exists today, either in the Congo as Mokele-Mbembe or in some even more hidden spot, it must be near the end of its time. If it is someday rediscovered before it disappears forever it will answer many questions. But even if it is never found it's a creature that has been capturing human imagination for over two thousand years and will probably continue to do so as long as humankind holds onto its sense of wonder.