Where the shape comes from
A rodent seen from below, its sharp teeth, whiskers, clawed feet and long trailing tail all packed into the strokes — a busier, more detailed picture than most animal radicals allow. It names the everyday creature in 老鼠, a mouse, a rat, and extends by a modern coinage to 鼠标, a computer mouse, and by simple compounding to 松鼠, a squirrel, literally a pine-tree rat.
汉字 filed under 鼠
No published character on HSK Guide is filed under this radical yet. 汉字库.