The horn is a brass instrument whose soft resonant timbre allows it to harmonize well with both brass and woodwind instruments. One of the unique aspects of this instrument is that the player keeps a hand in the bell while playing.
On a piano, however, those vibrations are initiated by hammers hitting the strings rather than by plucking or by moving a bow across them. So, the piano also falls into the realm of percussion instruments. As a result, today the piano is generally considered to be both a stringed and a percussion instrument.