Do we actually know anything about the Ents? I don't believe that there is anything in the Lord of the Rings about the actual history of the Ents. Did Tolkien write in length about he Ents?

I believe the origins of the Ents are mentioned in the Silmarillion. In the beginning. That they were created at roughly the same time (if we can speak of time back then) as the Eagles.
Hold on, I will look into it.

/me grabs his copy of The Silmarillion, and looks through the pages...

I found it, right were I expected it to be. On page 41 (at the end of chapter 2 [Of Aule and Yavanna]). On the end it says: "But in the forests Shall walk the Shepherds of the Trees".
Maybe some-one with more books by Tolkien, like maybe HOME, can shed more light upon this matter. This is the only thing I know about the Ents, not mentioned in LOTR.

Prior to the Elves awaking, Aule impatiently created the Dwarves. Although they were put into a sleep so they would not awake before the Elves, who were supposed to be the Firstborn, Yavanna was upset because she realised her husband had created something that would use her trees as fuel. With the aid of Manwe, she asked Eru for something to protect her trees. His response....

From the Silmarillion - of Aule and Yavanna



To me, this implies that Maiar spirits were summoned by Yavanna which entered the trees to become the ents. Although it does not specifically mention the spirits being Maiar, at the time only Valar and Maiar resided in Ea, so those spirits must have been Maiar.

In addition to the creation of the ents, Yavanna's thoughts also mingled with those of Manwe, and from these thoughts came the Great Eagles.

Further to the history mentioned in LotR, we also know that the Ents originally lived in Beleriand for they aided Beren's attack on the Dwarves from Nogrod, who had just killed Thingol.

It is a mystery what happened to the Entwives. They became estranged from the Ents and moved to the area of the Wold which became the Brown Lands. There they taught Men the art of farming and horticulture. Sauron, however, destroyed this land, burning and scorching it. Whether the Entwives were wiped out in this attack, Tolkien does not say. They have been reported as being missing ever since, however.

I always took Old Man Willow to be a Huorn.

What a lovely thought Rana. I think that in Arda the trees are sentient and alive. However most of them simply move so slowly that we cannot tell. The Elves held more reverence for the trees than Mortals did. Perhaps they knew what we don't. The Huorns perhaps were awoken by The Ents in some way, sped up perhaps. Another Tolkien mystery. Therefore Yavanna's part of the song was heard by Eru.

Another big mistery.... lovely to think that trees can hear, speak and even move!!! Sometimes we should think that trees and plants are really alive so please take care of them! I see daily how people behave as if they were only decoration or just a thing, they live and die as we do. It would be pretty beautiful to find an ent in the woods and chat a bit with it.... even if it takes hours just to say a whole sentence. Daydreaming....

when Gaffer Gamgee says that he had heard of walking trees near the Shire borders....      Could those possibly be the Entwives?  Or just a separated group of Ents?

I always just assumed this was probably some tree from the old forest.  I doubt Old Man Willow was the only Ent there, and even if he was, he may move around occasionally,the forest still moves and has some will in it, so I think he must still be herding the trees to a degree.

There was nothing that Illuvatar  did not put in to the music one way or another since He was all knowing and knew the beginning from the end. This was already done in His mind before the beginning of the music. All contingencies were covered and this segment of trees anyhow I think was always going to be alive as guardians. Because Illuvatar was well aware of the thought of everyone and everything and knew all about evil but planned that it should not win. The Ents were just one of the players as Bilbo said.

My theory is that the Ents are affected by the "alteration" of the world just like the Elves are. The Elves experience weariness as time passes in Middle Earth, that's why they eventually have to go to Valinor.

In the case of Ents they eventually turn into Huorns and then eventually into simple trees.