As Virumor mentioned, it is these spirits in the Chapter "Of Aule and Yavanna" which become the ents. Yavanna, concerned for her trees when the Dwarves are created by her husband, pleads with Manwe for something to be done. Manwe converses with Eru, and is shown how Yavanna's singing in the Great Music summoned spirits to enter the trees. It does not mention the name ents specifically, but it is obviously the Ents which are being writen about here.there is a passing reference to the trees by yavanna in 'of aule and yavanna' as creatures who sang to Illuvatar amid the rain and wind...the word ents not being used explicitly anywhere.
At the same time it is revealed to Manwe that his own singing, mixed with the themes of Yavanna, in a similar way summoned spirits that would become the Great Eagles.
As Ea was an empty place until the Valar and Maiar entered it, I believe the summoned spirits that entered the trees and became the ents must have been Maiar. In a similar way to which Maiar took on flesh to become the Istari, I feel others among them had taken form within the trees to become Ents. Like those that became the Istari, those which became ents no doubt also found their powers were weakened due to the burden of flesh.they are neither valar nor the first nor second born.
Only in the early writings were the Valar and Maiar able to procreate, and this ability was soon changed as JRR juggled his world around. The Ents were able to procreate, which does lead to an argument of whether they could be Maiar, but like Melkor, who lost power as he assumed a more corporeal body, thay may have found by leaving most of their Maian abilities behind when taking flesh, they could indeed have children.
I hope this is of some help to you.