Here's why:
* Two words combined: It's formed by combining two separate words, "sea" and "horse", to create a new meaning.
* Single entity: The combination refers to a single type of fish, not two separate things.
Compound nouns can be written as one word (e.g., "firefly"), two words (e.g., "sea horse"), or hyphenated (e.g., "high-school"). "Sea horse" is an example of a compound noun written as two words.