There is something odd about this turtle. Something not. quite. right.
Japanese depictions of tortoises occasionally have some odd features. While some can be written off as artistic license (such as dragon heads or lion feet), many paintings feature turtles covered in what seems to be hair or sporting many small tails. You can an example of this feature trailing behind the turtle above. These creatures are what are known as minogame.
The idea behind the minogame is that certain turtles have the ability or good fortune to become hundreds or thousands of years old. And these turtles, sitting like silent buddhas in their ponds, will naturally start grow seaweed or freshwater plants on their shells as they meditate. When they move this growth trails will naturally behind them like a coat (in fact, the term minogame is originally related to the word mino, or straw raincoat).
What I find interesting about this is that minogame are not entirely mythical. While it is uncommon, pond plants do sometimes grow on the shells of turtles. A simple observation of a quirk of ecology ended up spawning buddhist myths and depictions, eventually turning into a cultural touchstone. People were no less observant hundreds of years ago, but their reaction to what today is considered simply an interesting anecdote about ecology turned into something artistic and deeply symbolic.