Before the coming of the elves, Aulė a Valar(sub God) made the dwarves secretly, and they were allowed to exist. Another Valar who had made the forests worried that because the dwarves were made from stone that they would only love the things in the Earth and ignore or mis-use the forests on the Earth. She also feared the coming of men and what they might do to her forests. Finally a solution was found and the other Valor agreed.
She created Guardians for her forests. These Guardians were like trees but given the gifts of movement and speach. These were the first Ents and Entwives. After the coming of men, dwarves, and elves the Ents would only move against these other races if they destroyed forests in needless wars or did any great wanton destruction. Later in the course of many ages the Entwives were lost and the Ents were unable to have children. This resulted in many Ents declining and only Guarding small areas of forest and leaving the rest of Middle Earth's forests to the mercy of others. Another term used for the Ents was "Shepard of the Trees".