Huorns are much more dangerous than their more awake counterparts. They especially hate orcs, but any creatures that chop wood too freely are their enemies. They are vengeful, but their methods of exacting revenge are unspecified; people do not leave the forest if the Huorns do not let them. Huorns can create darkness to conceal their movements and are capable of moving quickly. They still have voices and can speak to the Ents, but unlike Ents, they do not seem able to speak intelligibly to other races.
Huorns are only directly mentioned in The Lord of the Rings as existing in the Fangorn forest, where they are roused by Treebeard, take part in the destruction of Isengard, and help the Rohirrim win an important battle at the Hornburg. They destroyed the Uruk-hai trying to escape to Isengard. The Huorns also form a barrier between Helm's Deep and Isengard, and also set a ring around Isengard, and destroy all the orcs who entered the forest. After the battles, the Huorns returned to Fangorn and are implied to have settled into full tree states.
It is possible that the trees of the Old Forest, especially Old Man Willow, were Huorns as well. This is implied by Treebeard's statement in response to Merry's question as to whether the Old Forest was similar to the dark spots of Fangorn forest,
"Aye, aye, something like, but much worse. I do not doubt that there is some shadow of the Great Darkness lying there still away north; and bad memories are handed down. But there are some hollow dales in this land where the Darkness has never been lifted, but the trees are older than I am."
He continues later, saying, "Aye, aye, there was all one wood once upon a time from here to the mountains of Lune, and this was just the east end."
In the MMORPG computer game The Lord of the Rings Online: Shadows of Angmar, huorns can be found in the Old Forest, Lone-Lands, and the Ettenmoors. They appear as twisted trees, with roughly eye-shaped crevasses in their trunks. Some of the huorns, particularly in Agamaur, are so motionless and tree-like as to resemble harmless scenery on higher graphics settings, a danger for the unwary player. They attack any player that lingers near them for more than a few moments. Huorns attack by hitting players with their roots, which they also use as legs.