Naruto

The very legend about a fox with 100,500 tails.

Actually, there is a mythical creature that is a fox with nine tails, and even more. And Yamato no Orochi (a snake, if I remember correctly), has seven heads and seven tails. And the Greek Hecatoncheires are also a good example.

in the body of a child, nothing but laughter is evoked.

Again, the plot about a demon in the body of a child is quite typical. The Japanese simply placed their mythical creature in a child instead of Western demons.

So, I don't see anything strange in such a plot; on the contrary, it is quite banal.