The Huorns: History (p2)

The Huorns mostly stood as dark trees in the deepest forests, gnarled and unmoving, yet watchful. When aroused in wrath they moved swiftly as if wrapped in shadows, falling on foes with deadly and merciless strength.

The tale of the War of the Ring tells of how, with the Ents, the Huorns marched on Isengard like a great forest, and how, under the direction of the Ents, they exterminated the entire Orc legion in the Battle of the Hornburg.