What an improbable animal is the seahorse! With a horse's head, a possum's tail, eyes like tiny glass fishbowls and fins that wave like chiffon frills, the shy creatures have intrigued and delighted us since ancient times. But these thoroughbreds of the sea are now in danger of their lives. Ground up for medicines, sold, sightless and stiff, as souvenirs, captured alive for home aquariums, seahorses in many parts of the world have a hard ride ahead of them. A combination of public awareness and aquacultural research-some of it being conducted in this country-may yet turn the tide for these fascinating animals.