The Vala associated with the creation of the Ents is Yavanna, the guardian of growing things. She feared that the coming Children of Ilúvatar would destroy the forests and trees she loved.
After learning that Aulë had created the Dwarves, Yavanna asked Manwë whether the trees could have protectors of their own. In response to her concern, Ilúvatar allowed the Ents to awaken as shepherds of the forests. Yavanna therefore prompted their coming into the world, although she did not personally shape them as Aulë shaped the Dwarves.
Yavanna was one of the Valar and the wife of Aulë. The Elves called her Kementári, meaning “Queen of the Earth.” She was also associated with the Two Trees of Valinor, Telperion and Laurelin.
Aulë is the tempting alternative because he created the Dwarves, while Manwë delivered the decision concerning the Ents. But the Vala responsible for requesting and inspiring their creation is Yavanna.