What manner of creature or being Old Man Willow is exactly is not made clear in the book. It is possible that he may be some form of Ent, or more likely a Huorn, the latter being a more wild and tree-like Ent. Unlike the Ents that appear later in the book, Old Man Willow does not appear to be able to walk, but draws his victims to him instead (though it is suggested that some of the trees in the Old Forest do move). Later, Treebeard explains that Huorns tend to be angry and vengeful, a description that fits Old Man Willow well.
The Ents and Huorns were probably influenced by the sentient trees of many real-world mythologies. These include talking trees that might tell the future, or help a person look for Leprechaun gold. Alexander the Great and Marco Polo were both said to have visited a prophesying Indian tree. Oak and Rowan trees might be conversed with by Druids. In Greek mythology, spirits called Dryads were said to inhabit trees, with which they shared a deep connection; a dryad would often die if their tree was cut down. Tree spirits are common in other cultures too, such as the Japanese kodama. Trees, magical or not, are sacred in many different cultures.
Talking, walking, and magical trees can be found in many other fictional works, as can dryads and tree spirits. Talking, dancing trees and their tree spirits appear in the Narnia books, written by Tolkien’s friend C. S. Lewis. Dryads are also mentioned in Milton’s Paradise Lost. In The Wonderful Wizard of Oz, The Forest of Fighting Trees attacks the Scarecrow. Kodama appear in the animated film Princess Mononoke.