Pirahna is a Badnik from the Marina Madness area in Knuckles' Chaotix. Since there are no water obstacles in the Zone, Pirahna floats above the boats and spits out exploding mines. Although Pirahna doesn't appear in any Sonic the Comic strips, he is featured in the Badnik Alert! section of the Knuckles Knock-Out Special. It is fairly dangerous, given a Danger Rating of 3/5.
Pirahna is actually spelled Pirani in the actual game. The actual Badnik name is a misspelling anyway, since the real fish is called a "piranha".