Some Piranha species are plant eaters.

A Piranha Plant is a plant like creature and a minion of Bowser. They commonly appear in Warp Pipes and fear fire. In MK:DD!!, they lashed out at drivers who came too close to them. They were stunned by launcing a projectile at them.

Herbivore means plant eater. Piranhas aren't plants. No herbivore would eat a piranha.

cant beat it

There is a red belly piranha which eats meat and also a pacu which eat plant and veggie and which is a type of piranha