Piranhas was added in Beta 51.0 as a replacement for an older version of Starwalker Bird (more specifically, its Beta 34.0 version). Previous Versions can be found here.
As of Beta 53.0, it still greatly matches StarBird as a 6G 3/4 Haste monster (which is +1 HP from StarBird) with the ability to deal 1 DMG to a random enemy monster after it has attacked and killed another one. This effect can trigger from its own ability, allowing it to clear a board of 1 HP monsters. However, StarBird had Ranged among that, which has since been removed. In its place, Piranhas has Dodge (1). It's also an Epic rather than a Common.
As a result, while the user can only start with 2 rather than 3 and is unable to use this in a deck with the new StarBird and Starwalker, it has more HP and can avoid an enemy attack once if it didn't attack a monster on its first turn.