What else but the infamous piranha! These ferocious fish live in rivers in tropical Central and South America. Piranhas are only about twelve inches long (30 cm) but a group or a shoal of hundreds attacking together can overwhelm and devour a much larger animal in just seconds! Their weapons are their extremely sharp triangular-shaped teeth which they use to hack the flesh off their unfortunate target. Although piranhas are more known for their aggressiveness, some species of this fish, however, are relatively harmless and feed only on plants.