Pokechests are chests that look like pokeballs. They disappear after giving you items. They comes in three types: Pokechest, Ultrachest and the Masterchest. They are not craftable. Some items that can be found are Evolution stones, TM's, and various kinds of Pokeballs, Potions, and held objects. Look closely because some Pokechests are hidden (invisible), although they emit white particles. You can also find them more easily with a Item Finder.
The Poke Chest and the Ultra Chest rarely spawn usually in any biome in a world. The Master Chest can be found only in forest and plains biomes in large trees.