Treebeard is called the oldest of the Ents in J.R.R. Tolkien’s The Lord of the Rings.
His Elvish name is Fangorn, and he is the principal guardian of Fangorn Forest. Merry and Pippin encounter him after escaping from the Orcs who captured them. Treebeard initially regards the hobbits as an unusual and difficult-to-classify kind of creature, but their account of Saruman’s destruction of trees helps prompt the Ents to action.
The Ents are shepherds of trees, an ancient people created in the mythology of Middle-earth. Treebeard leads the Entmoot, a gathering that debates whether the Ents should oppose Saruman. Their eventual assault on Isengard becomes one of the major turning points in the War of the Ring.
Quickbeam is another named Ent, while Bregalad is his Sindarin name. Tom Bombadil is an entirely different figure, so he is not one of the Ents or the oldest of them.