"We are tree-herds, we old Ents. Few enough of us are left now. Sheep get like shepherd, and shepherds like sheep, it is said; but slowly, and neither have long in the world. It is quicker and closer with trees and Ents, and they walk down the ages together. For Ents are more like Elves: less interested in themselves than Men are, and better at getting inside other things. And yet again Ents are more like Men, more changeable than Elves are, and quicker at taking the colour of the outside, you might say. Or better than both: for they are steadier and keep their minds on things longer."
Ents, called Onodrim (Tree-host) by the elves, are a very old race that appeared in Middle-earth at the same time that the elves did. They were apparently created by Eru Ilúvatar at the behest of Yavanna after she learned of Aulë's children, the Dwarves, knowing that they would want to fell trees. Ents were envisioned as Shepherds of the Huorns, to protect the forests from orcs and other perils. They are the siege units of Lothlórien but can also be summoned for a short time through the Rohan faction. They are strong against most units and structures but very vulnerable to fire.