Here's why:

* Complex Structure: Seahorses have specialized organs and tissues like a brain, heart, gills, and fins. These structures are all composed of many different types of cells working together.

* Multicellularity: Multicellular organisms are defined by having multiple cells organized into tissues, organs, and organ systems. This is a defining characteristic of animals, including seahorses.

In contrast, unicellular organisms (like bacteria and amoebas) are made up of only one single cell.