The main characteristics of fish: fish live in water all their lives, and their bodies are spindle-shaped, mostly covered with scales. They breathe with their gills and swim with their fins. Fish has a spine composed of many vertebrae, which has the functions of heat preservation and protection. Body temperature usually changes with the change of environmental temperature, so hippocampus belongs to fish. Amphibians live an amphibious life, with larvae breathing through gills and adults breathing through lungs and skin. Mammals have the characteristics of viviparous lactation. Reptiles are covered with horny scales or nails and breathe with their lungs. Their body temperature is not constant and will change with the change of external temperature, so seahorses belong to fish.

So choose: B.