B. Shrimp is a long-lived animal living in water, belonging to arthropod crustaceans, not fish;
C, squid is soft, has a mantle, and is a mollusk, which does not meet the meaning of the question.
D. Hippocampus lives in water, has fins, breathes through gills, and stands upright in the water, so it belongs to fish, so it meets the meaning of the question.
Therefore, choose: d.