The Mineral Fish. Simply put, Minecraft sprites edited to be fish. However, it goes deeper. Mifernite! The Kumquats! The MF IRC channel! However, we’re not talking about any of that stuff.
We’re talking about the origins of the Mineral Fish.
In real life
In simple terms, the Scratch user -gge made Copper Fish after someone on the Minecraft forum topic said something about copper, a new ore added in the then-new 1.17 update, resembled salmon, and how they mistook it for a “raw salmon nugget” and after LandonHarter made this image:
From there, -gge started making other fish, as part of the original batch, and then from there, other users started making their own fish, which are now known as “unofficial fish”.
In the lore
The Mineral Fish lore, for how all over the place it is, explains how the fish were created and discovered in terms of lore.
So, one day, a pebble falls into a lake of Mifernite (a liquid that can turn anything into a Mineral Fish), which creates Pebble Fish. While Pebble Fish was trying to figure out what had just happened to them, a landslide happens and a large stone falls into the lake, creating Stone Fish. Pebble Fish, realizing what the liquid does, explains what happened to Stone Fish and they start experimenting with the liquid, throwing in many kinds of materials, like copper and wool, into the Mifernite lake, creating the original batch of fish.
Later on, -gge stumbles upon a portal to Mandorok (the Mineral Fish world) and ends up in Dantelkrok (a continent that holds portals to many other worlds and dimensions), and discovers the Mineral Fish. He later goes back to Earth and explains his findings to other people, and a Wiki was created to hold all of the information. And that’s how the fandom was created.