Old Man Willow

Old Man Willow was a willow in the Old Forest from which much of the Forest's hatred of walking things came. He might have been an Ent who had become tree-like, or possibly a Huorn, as the Old Forest was originally part of the same primordial forest as Fangorn.

Tom Bombadil had power over Old Man Willow, and checked the evil as much as he could, or was willing.

Appearances
In The Fellowship of the Ring book, Old Man Willow cast a spell on the hobbits (Frodo, Sam, Merry and Pippin), causing them to feel sleepy. Merry and Pippin go and lean against the trunk and fall asleep, while Frodo sits on a root to dangle his feet in the water, before he also falls asleep. The tree then traps Merry and Pippin in cracks of its trunk, and tips Frodo into the stream. They are saved by the timely arrival of Tom Bombadil who 'sings' the ancient tree to sleep.

Although this scene did not appear in the 2001 movie adaptation, a very similar episode with hobbits being swallowed by a tree was included in the extended DVD edition of the second film where Merry and Pippin are attacked by a Huorn in Fangorn forest. In this interpretation Tom Bombadil's lines are spoken by Treebeard.