The tale of the War of the Ring tells of how with the Ents the Huorns, like a great forest, marched on Isengard, and how under the direction of the Ents they exterminated the entire Orc legion in the Battle of the Hornburg.
These were wild wood spirits who were bent on the destruction of all who threatened the forests. They were dangerous to all on two legs unless they were protected by Ents.
Huorns were ancient and long-brooding, and some were black-hearted and rotten. At least one such sentient tree spirit inhabited the Old Forest by the banks of the Withywindle River; he was called Old Man Willow. As the Old Forest and Fangorn were the last remnants of the most ancient forests in Middle-earth, Old Man Willow wished to prevent further inroads into his realm. He held all the Old Forest in an enchantment by the power of his song and led all travellers to him, where with his limb-lithe roots and branches he ended them.
In the book, The Two Towers (Book) the Huorns are witnessed to march to Isengard with the ents. After Isengard empties of orcs most of the Huorns chase them and kill them after the Battle of the Hornburg. After the sack of Isengard they are again seen but this time they surround Isengard so that any orcs that escape are killed eventually. After this brief appearance, the Huorns go back into the forests. The tactics used by the Huorns were quite effective as the Huorns only attacked when all the enemy orcs would be in their midst which denied them the chance of escape and also since they waited no orcs would be able to see the carnage from outside and escape.