"The Natural History of the New Japan Islands" writes that over the years, a number of historians, biologists, and linguists have puzzled over the etymology of the name "minoshiro", and come up with a few interesting theories.
An old accepted explanation was that its name came from the fact that looked like it was wearing a raincoat.1 But the book doesn't say what kind of raincoat, and I've never seen one before, so I have no idea whether this explanation is accurate or not.
Another reason came partly from the cape-clad appearance, and partly from its white color, combined with the belief that the souls of the dead lived within it.2 Also, the fact that the minoshiro is usually terrestrial, but returns to the sea to lay eggs was a plausible origin for its name.3 A later explanation was that the red and yellow eggs that it laid in clumps of seaweed or coral resembled ornaments in the palace of the Dragon King.
Another unofficial reason came from the fact that the when it faced an enemy, the minoshiro's tail will bristle and stand straight up, like a shachihoko4 found on the roofs of castles in the ancient past. They named it after the castle in Mino, but later research showed that the it was Nagoya Castle that had shachihoko, which was in the neighboring province of Owari. After that discovery, the explanation lost its appeal.
There are also numerous stories saying that "shiro" is the name Shirou shortened. Since Shirou was just over a meter tall, he was called Minoshirou ("mino" is three times the length of a standard-width cloth, around 108 centimeters). A different story said that once he met a snake-like creature with numerous tentacles, which also gave him the name Minoshirou.5 The stories are varied and hard to get a grasp on.
Still on the topic of Shirou, one old folktale says that he was cursed by a white snake and turned into a minoshiro. Since other details of the story were lost, there is no way to prove its authenticity.
Personally, I think any of the stories are possible. At least it is much easier to understand compared to the etymology of the name of the toads that are everywhere on Mt. Tsukuba. In the book it says that "it uses powers to draw in and devour insects". Who would believe the idea that toads have canti?
Another mystery surrounding minoshiro is that it's not mentioned in most ancient texts. Even though many of the texts from over a thousand years ago are off-limits, the word "minoshiro" is still nowhere to be found in available texts. That means that minoshiro were discovered within the past couple hundred years, but an entirely new creature evolving within such a short timespan is unthinkable.
Actually it's not just minoshiro. In the years between civilization from a thousand years ago and today, there was a mass extinction of fauna. That past species all went extinct is unusual but not completely unexpected, what was surprising was the sudden appearance of minoshiro and hundreds of other species as if they had come out of thin air.
One hypothesis that seeks to explain this phenomenon has been garnering attention lately. It says that their evolution was driven by the collective human unconscious.
But that seems a bit extreme. Just recently, it was determined that minoshiro descended directly from a species of sea slug called the indica nudibranch that lives around the Boso region. Although it's hard to imagine a 30-centimeter long sea slug evolving into something as big as a minoshiro, when you look at the {raincoat-like protruding gills} you have to admit that there is a definite resemblance. If the sea slug is really the ancestor of the minoshiro, and they share the same name1, then that is supporting evidence for the first two theories that were mentioned earlier. But I think more research is still needed.
The reason I'm mentioning all this is that in order to understand what I'm talking about when I get to the part about meeting the false minoshiro during summer camp, you need to know what a real minoshiro is.
