Piranha

The Piranha are enemies that can be found in water pools of the Jungle, Underground, and Cavern layers. They have an increased spawn rate in the Jungle. They swim in a straight line left or right until hitting a wall, where they flip directions. They will swim towards the player if he/she is submerged in water; otherwise they pay no attention to the player. It moves its tail back and forth and swims in a slight zig-zag.

When your world enters Hardmode, Piranhas become mostly overridden with Angler Fish, however; piranhas will still be common in Jungle biomes.

History
Piranha Piranha Piranha Пиранья