Treebeard, also known as Fangorn, was the oldest of the Ents, a tree-like being who was a sort of "shepherd of trees". Very tall and stiff-limbed, with bark-like skin and leafy hair, he befriended Meriadoc Brandybuck ("Merry") and Peregrin Took ("Pippin") after they escaped from Orcs. He lived in Fangorn Forest, which was named after him (or he after it).
Like most Ents, Treebeard took a long time to make up his mind. He repeatedly spoke of not "being hasty". Treebeard is described as the oldest being in Middle-earth, which is the cause of some confusion as Tom Bombadil is also called "Eldest". Treebeard however said that there were trees in Fangorn that were "older than he." The general consensus is that Treebeard was the oldest living thing in Middle-earth, as Tom Bombadil is not exactly a member of any race.
It has been theorized that many of Treebeard's mannerisms were based on Tolkien's friend C.S. Lewis, a loud, bombastic man known for his powerful stride and overwhelming presence.
Treebeard was inspired in many ways by C.S. Lewis including his speech patterns and some aspects of his personality.
Portrayal in Adaptations
In Peter Jackson's The Two Towers (2002) and The Return of the King (2003), Treebeard is a combination of a large animatronic model and a CGI construct; his voice is performed by John Rhys-Davies, who also plays Gimli.
In the movie The Two Towers, the Ents at the Entmoot initially decline Merry and Pippin's request for help. Treebeard is carrying the hobbits through the forest, when Pippin gets an idea. He asks Treebeard to carry them towards Isengard, because that is the last place Saruman would look for them. When Treebeard gets close to the forest's edge, he sees the devastation wrought by Saruman's orcs. The trees have all been chopped down to serve as fuel for Saruman's war machine. Treebeard then makes a momentous decision, and calls for the other Ents. They attack Isengard and manage to flood it.
In the book, Merry and Pippin speak at the Entmoot, helping Treebeard convince the other ents to attack Saruman without any such trickery.