Treebeard

Treebeard is one of the oldest still-living Ents. He lives in Wellinghall, located in a secluded glade at the western edge of Fangorn, near the source of the Entwash. Bringing Harle to meet Treebeard is the objective of the "Harle and the Entwives" quest. Treebeard will also reward anyone who can find and kill the creature that is poisoning the forest.

An old, wise tree with penetrating green eyes stands here.
>examine Treebeard
A man-like creature, clad in green and grey bark sturdily towers fourteen feet
into the sky before you. His knob-knuckled hands have long agile fingers, and
the great arms are covered by a brown smooth skin. A tall head, with eyes shot
with an inner light of wisdom deep within their wells, is upon the wrinkled
hide covering the core. Further downward, flowing from the roots of his face
grows a twiggy beard which turns mossy at the ends. Both his huge feet carry 
seven toes each and seem to root well in the ground.

In LotR

Treebeard found Merry and Pippin when they were lost in Fangorn. When he learned from them that Saruman was cutting down the forests, he led the Ents in an assault on Isengard, destroying and flooding most of the fortress aside from the central tower, where Saruman was trapped.