The Eastern Forest is an area that was visited by Kirby and friends in the Kirby: Right Back at Ya! episode War of the Woods. It is located just east of Whispy Woods Forest, and is the home of his old friend Acore, as well as the Animal Friends.
Acore is the main reason why King Dedede wants to cut down the Eastern Forest before cutting down Whispy Woods' forest too, therefore the King tried to cut him down once, but Kirby and the gang managed to stop him from doing so.