Piranha

Left alone it patrols a pool in flat lines, flipping around whenever it meets a wall. While you are on dry land it does not care that you exist. The moment you submerge, everything in the water turns and comes at you at once, so a jungle pool you had been treating as scenery becomes a fight the second you decide to swim through rather than around. They also turn up twice as thickly in the jungle as in the caverns.

Dragged or knocked onto land it thrashes about erratically and still hurts anything it touches. A hook or a knockback weapon that hauls one out of the water has made its movement harder to read rather than made it safe.

Most of them get replaced by Angler Fish, which hit far harder and behave much the same. A jungle water route you crossed comfortably before hardmode will kill an underprepared character afterward, so scout it again instead of assuming. If you only want the banner, a Piranha Statue on a timer produces them on demand and saves you the swimming.