Treebeard

Treebeard, the oldest of the Ents, and, in fact, the oldest of all living creatures of Middle-earth, is the guardian of Fangorn Forest. Treebeard is merely a common translation of Fangorn – his true name. Treebeard, as well as all other Ents, are the first creations of the Vala Yavanna. He is first encountered in the First Age, when he rescues Beren from an Orc-camp. In the Third Age, Fangorn found Merry and Pippin on the edge of the Forest and, giving in to their entreaties, summoned a gathering of all Ents, the Entmoot.

This first meeting is so described in the LotR:

They (Merry and Pippin) found that they were looking at a most extraordinary face. It belonged to a large Man-like, almost Troll-like, figure, at least fourteen foot high, very sturdy, with a tall head, and hardly any neck. Whether it was clad in stuff like green and grey bark, or whether that was its hide, was difficult to say. At any rate the arms, at a short distance from the trunk, were not wrinkled, but covered with a brown smooth skin. The large feet had seven toes each. The lower par of the long face was covered with a sweeping grey beard, bushy, almost twiggy as the roots, thin and mossy at the ends. But at the moment the hobbits noted little but the eyes. These deep eyes were now surveying them, slow and solemn, but very penetrating. They were brown, shot with a green light.

 Eventually the Ents of Fangorn, brimming with anger at the axes of Isengard, leave the eaves of their forest and come down to Orthanc, where they pull down its walls, ruin the shafts and mines and flood the valley, thus imprisoning Saruman in his own fortress. Treebeard becomes keeper of the keys of Orthanc. He does not leave Fangorn Forest, his domain, for many years to come, and under his care it grows and expands until it becomes one of the greatest forests of Middle-earth.