Making a hippo costume may seem like a huge task, but most animal costumes can be thrown together rather quickly. In fact, it's entirely likely that you already have most of what you need to make a hippopotamus costume at home. Best of all, you don't need any special craft skills to execute this costume; as long as you can cut and paste, you can make this costume.
Cut the pink felt square into a circle about 12 inches in diameter. Cut the circle in half so that you have two symmetrical sides.
Line the two sides of your circle up on the middle of the one piece pajama. Glue the two sides to the pajama so they appear to be one circle, but don't put any glue on the zipper.
Use strands of gray yarn to braid together for a hippo tail about 12 inches long. Knot the top and bottom of the braid to secure it. Hook a safety pin through the top knot and attach it to the back of your costume.
Glue the two googly eyes to the top front of the gray baseball cap. Cut teeth out of the white felt and glue the teeth to the edge of the front bill of the hat so that they hang down.
Wear the costume with a pillow zipped up under the front midsection. Use a belt to secure the pillow to your waist.