The seahorse is one of nature's most startling creations. For the ancient Greeks and Romans, who found them washed up on shore after storms, the only explanation for such an astonishing form was a mythological one: these creatures pulled the chariots of Neptune. The seahorse, even to us nowadays, seems out-of-this-world. It is a fish, but has the head of a horse and wears a crown. Its fins move so fast they are almost invisible. It has a tail, but...