What are about percussion instruments?

There are many instruments which can be played in a percussive manner but are not classified as a percussion instrument. Just because an instrument is not classified as a percussion instrument does...

A Piano

Yes and a piano is also percussion because when you press the keys a hammer hit a string and a hammer strikes a wire producing the sound.

Percuss is a medical term, to strike. When the doctor percusses your chest, she is striking it. So a percussion instrument is struck, mainly. There are others that you can shake, rattle or stroke,...

Yes. They were created by early humans.