Piranha

The Piranha 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 are still common in jungle biomes.

History
Piranha Piranha Piranha Пиранья