Middle-earth seems to be a period where all our present legends can be traced to real life peoples and events. Elves for example are a real race. So the Ents must too be an actual precedent for a legend. And what legend might that be? Why the legend of the Green Man, of course, or Jack in the Green, green Jack, Green George One can still see some carvings of him in ancient churches. Some resembled a human face with branches of leaves sprouting out from his mouth and nostrils (shades of Treebeard?) some simply a pair of eyes looking out mysteriously from a mask like covering of leaves. Most writers has remarked on the feeling of antiquity that the eyes gave. Again I point out the comments by whoever that saw the Ents eyes, the Rohan riders, the hobbits etc. The Green Man has been associated with Christ, with paganism, with the wild Woses of Britain (not Middle Earth's). Its origin would probably be traced back to the biblical fertility gods like Baal. The Ents and their wives supposedly taught the people of Middle-earth agriculture, that important step away from the evolutionary stage of nomadic hunter gatherer and towards urban life. This might be Tolkien's way of explaining the ancient fertility worships and cults of gods like Baal and the Celtic barley god. Incidentally the Green Man has always stayed true in one aspect throughout his long history, and that is he is male. And why Tolkien's emphasis of the estrangement of the Entwives from the Ents? Well from the Stone Ages to the Greek Mycenean ages, there has been worship of Nature as a goddess, a female. These people might have been taught agriculture by the Entwives. Most relics of the male fertility cults are mostly penis shaped . But we do get a good idea of how the Entwives would look from the Voluptuous 'Venus' statues that has been carved by from Neanderthals to the Cro-magnons. In short, the Ents and the Entwives might be Tolkien's way of explaining the male-female dichotomy of the earlier religions.
I've stated earlier the Green man has been associated with the Woses and antiquity, being called at one point, woodwoses. This goes to show of the Middle Age people's awareness of the origin of the Woses, the 'wild man of the Forest' as a kind of ancient race earlier than humans. The Woses are Britain's version of the world wide wild man myth; the 'orang pendeek' of Malaysia, the 'orang sedapa' of Indonesia, the Bigfoot of the USA. Like the Green Man, There still exist carvings of them in churches. They look like hairy humans. The Woses of Tolkien most agree in appearance with the modern reconstruction of the Neanderthal. 'Squat' means heavily built besides short. Tolkien's first loves were the physical sciences so I would assume he was aware of the fossils of early man. If I'm right, then he must also know of the pteronodons and the mammoth. That would probably explain the 'mûmakil' and the winged Nazgûl steed