Huorns are creatures much like Ents, although they do not appear to have been truly sentient. It is not clear if Huorns are Ents that have become tree-like, or trees that have become Entish. Perhaps both varieties exist. Either way it is obvious they are part way between trees and Ents.
Huorns are only directly mentioned in The Lord of the Rings narrative as existing in the Fangorn Forest, where they are roused by Treebeard and help the Rohirrim win an important battle at the Hornburg. It is possible that the trees of the Old Forest, especially Old Man Willow, were Huorns as well.