Cut out head, tail, and fin shapes from paper plates.
Tape an uncut paper plate (this will be the body) to the back of the head. Roll up a small piece of tape—sticky side out—and sandwich it between the back of the head and the front of the body to keep the head in place.
Cut out an eye shape from a paper plate scrap and tape it in place, white side up. Use a marker to color in a pupil. Also, use markers to add spots or stripes to the seahorse.
Optional: Create an entire undersea world in your child's room by making several brightly colored seahorses. Punch a hole in the top of each seahorse head. Tie a string through the hole. Suspend the seahorses from the ceiling. Tape crepe-paper streamers from the ceiling for the seahorses to drift through.