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 film adaptation of the Two Towers Huorns are not expressly mentioned or even seen except in the extended edition. Here they are witnessed by Merry and Pippin just before the Ents' attack on Isengard, following the Uruk-Hai army to Helm's Deep. They are later seen after the Battle of the Hornburg, when the remaining Uruk forces flee into a forest that had not existed before the battle. The Huorns attack only when all the Uruks are within their numbers, violently crushing the entire army with their branches.