Mouse Droid was an NPC, a droid onboard the Peace Moon.
A New Generation[]
While Greedo/Fake!Han Solo and Adam Lars/Luke Amidala/Luke Starkiller were disguised as Clone Troopers onboard the Peace Moon with Chewbacca, Han told Chewbacca he should growl or something because it would look suspicious otherwise. Chewbacca reluctantly pretended to growl, which was enough to scare a Mouse Droid who let out a squeak and fled away from them.
Later, the Mouse Droid was destroyed along with everything and everyone else on the Peace Moon when Luke blew it up.