Toxiculum

"Toxiculum is a genus of gen 2 fish that has developed a completely new form of defense. It has developed a neurotoxin that acts in a similar way to tetrodotoxin, suffocating whoever tries to eat it. Toxiculum is distasteful to predators and is slower than the Yufish, although has developed additional fins for steering. Toxiculum can be easily identified as it has a vibrant pattern with bright fins, which it uses as a warning signal towards predators."

-DinoCoat

It has taken a similar niche to today's blowfish, using poisons to deter predators from consuming it. Toxiculum is descended from the Yufish of generation 1. Its blue, white, and yellow coloration is somewhat based off of the lagoon triggerfish. The fins are jagged, and arm fins have developed. Toxiculum also seems to have a horn-like appendage at the front of its head, most likely used as another form of showing off, and possibly even mating rights.

It has also been mentioned by the creator that Toxiculum's neurotoxin is not actually fatal, but actually causes discomfort and throat-swelling for the predator. It is not said that future forms will be as nice though.

Trivia
Toxiculum evolves into Coronaicthys in Gen 3.