Tolkien calls the tree-like people who march against Isengard Ents. They are ancient shepherds of trees, led by Treebeard, and their attack is one of the decisive events in The Two Towers.
The Ents gather at an Entmoot after Merry and Pippin meet Treebeard in Fangorn Forest. At first Treebeard is reluctant to involve his people in the war, but the damage caused by Saruman’s Orcs and the destruction of the forests change their minds. The Ents then assault Isengard, break its defenses, and flood Saruman’s stronghold.
Ents are distinct from Huorns, a related and more mysterious group of tree-like beings. The Great Eagles are intelligent allies but are birds, not trees, while the Maiar are powerful spiritual beings such as Gandalf and Saruman. Tolkien drew the word ‘ent’ from an Old English term meaning giant or creature.