I couldn't find in a quick skim of Silmarillion any mention of the Ents fighting in the battles of the First Age, so maybe BarrowBlade can pitch in there. I have a vague memory of what he/she's talking about, but haven't looked at the Silmarillion in about 25 years.
So I just looked at it to see if I was right about the Ents' creation, and here is what I found:
In Chapter 2 of the Quenta Silmarillion itself (not the Creation section, after all) is recounted the creation of the Dwarves and the Ents and the Great Eagles by Aule and Yavanna and Manwe.
Skipping past the Dwarves here -- the chapter essentially fills in the tale of free-willed creatures before diving into the story of the Elves -- Yavanna feared the harm that would come to all her creations (the plant world) once sentient beings roamed Middle-earth: hewers of wood and harvesters of fruit. She had snuck into the song of creation the idea that the trees had spirits of their own, and so showed Manwe that they deserved defenders; he had not noticed her contribution before, but he replied that he too had added a few movements that gave the animal kingdom an independent dignity. So Eru sent "spirits from afar" to inhabit/"go among" the animals and the plants, and serve as their guardians and protectors against the Children (Elves and Men). The Eagles answer to Manwe (God of the air and winds) and guard the animals; and the Shepherds of the Trees (Ents) answer to Yavanna (Goddess of Plants) and guard the trees and shrubs.