Find one of the Skeleton Dogs; the easiest place to do so is the Pumpkin Hill stage with Knuckles, and combine it with your Chao. With either Knuckles or Rouge, start digging around in the Chao Garden. Eventually a pumpkin will appear. If you give it to the Chao mixed with the Skeleton Dog it will put it on its head. If you want to take it off, either throw or hit your Chao.