There go Koishi's dreams of being a mermaid.

Haha, Koi fish. Clever.

I lol'd.
I had to take a closer look at this since I was not able to figure out what the hell this was.

Somewhat nostalgic, this. There was an episode in The Magic School Bus series where the kids turned into tuna with human faces.

adesa said: FINDING:

Behold!
During my travels through the lakes of Gensokyo, I stumbled upon the elusive KOI-shi.

This elusive fish are very rarely seen in the wild. The rarity of these fish are caused by their ability to manipulate the subconscious, causing itself to vanish in our eyes.

Despite the challenges, I managed to snap a photo of it in its natural habitat, with the help of the Optical Camouflage I borrowed from the Kappas.

Not only I managed to snap a photo of it, I snapped it during one of the most important moments of a KOI-shi's life: finding "the hat"

BEHAVIOR:

Upon reaching adulthood, KOI-shis began a journey in search of the hats. Once they find the hats, they lay their egg inside it. Each KOI-shis can only lay one egg, that explains their elusiveness.

The baby KOI-shis grow inside the hats and use it for home and protection, a behavior commonly seen in hermit crabs (don't ask).

MUTATION:

When they outgrew their hats, KOI-shis are ready to leave the safety of their hats and began a journey of their own. However, some baby KOI-shis grew attached to their hats, even to the point calling them "Mr. Hat".

These KOI-shis abandoned their rite of adulthood, thus grew irresponsible and self-centered. They will do anything they want, where they want and how they want. The mutation doesn't stop right there, it even changed their metabolism. These KOI-shis turned into ravenous carnivores and will kill and eat anything, including their own kind.

You can easily distinguish a mutated KOI-shi by their wide mouth, filled with fangs, their bulging eyes that glows and their pale complexion compared to normal KOI-shis.

....

....

This is what happened when you lacked sleep....

So,KOI-shi that can only lay one egg in their life and not only have to survive their natural predators,compete for food with another KOI-shis and other animals,have to avoid mutation and their mutated kin,thus basically doom the species to extinction,is only "elusive"?They must be more common than water in the past to even exist today.