Piranhas are a creature that swarms. Not the kind of thing a Beastmaster would bend to his will and service. I don't think it needs to be a Beast simply due to its being animalian.
Of course, if one really wanted to nitpick here, it could be pointed out that real-life piranhas are not dangerous at all. Their supposed ability to reduce a cow-sized animal to bones in a matter of seconds is pure fantasy. http://en.wikipedia.org/wiki/Piranha#Myths Not that there's anything wrong with fantasy in a game where we can unleash a Chimera, Cockatrice, or Genie on one another, mind you. =D