Flying Fish

Rain is the only requirement. Any surface biome, any point in the game, and the sky fills with them for as long as the weather holds. Nothing summons them and nothing gates them, so a storm in the first days of a world can be a genuine problem.

They drift toward you the way an Eater of Souls does, slowly enough that you can walk away from a single one on flat ground. Water is no escape, since they swim into it freely, which catches out anyone who ducks into a lake to lose them.

The detail that matters is that they need to see you. A roof overhead, or any wall between you and the swarm, drops them completely. During a long storm, a covered walkway between your base and whatever you are building pays for itself in blocks.

Real flying fish glide on stiff fins instead of flapping, and water is where they belong. Terraria reversed both, then made them hungry.