Flounder

The Flounder is a Pre-Hardmode enemy that spawns in the Sulphurous Sea. It initially stays invisible on the sand, waiting for the player to come near or hit it, in which it will then become visible and hostile. It attacks by slowly swimming towards the player while spraying sulphuric mist projectiles at them.

Trivia

 * In real life, flounders are a group of flatfish species. They are found at the bottom of oceans around the world; some species will also enter estuaries.