Ents, called Onodrim (Tree-host) by the elves, are a very old race that appeared in Middle-earth at the same time that the elves did. They were apparently created by Eru Ilúvatar at the behest of Yavanna after she learned of Aulë's children, the Dwarves, knowing that they would want to fell trees. Ents were envisioned as Shepherds of the Huorns, to protect the forests from orcs and other perils. The Elves have tales of teaching the trees and the Ents to talk: although the Ents were sentient beings at the time, they did not know how to speak until the Elves taught them. Treebeard spoke of the Elves "curing the Ents of their dumbness" that it was a great gift that could not be forgotten ("always wanted to talk to everything, the old Elves did").
In The Two Towers, Chapter 5, "The White Rider", Gandalf calls Treebeard, "...the oldest living thing that still walks beneath the Sun upon this Middle-earth." In The Return of the King, Chapter 6, "Many Partings", Celeborn addresses Treebeard as "Eldest."
Ents are tree-like creatures, having become more and more like the trees that they herded. They vary in traits, from everything to height and size, colouring, and the number of fingers and toes. An individual Ent more or less resembles the specific species of tree that they typically guard. For example, Quickbeam guarded Huorns and thus looked very much like a Huorn (tall and slender, etc.). Ents are very strong, as recounted by Merry and Pippin: "their punches can crumple iron like tinfoil, and they can tear apart solid rock like breadcrusts." In the Third Age of Middle-earth, the forest of Fangorn was apparently the only place Ents still inhabited, although the Huorns may still have survived elsewhere, as in the Old Forest.