A mouse droid, or an MSE-series as some people like to call it, was a really tiny and extremely annoying droid found on the Death Star. They traveled at the speed of 90 km per minute and would usually steal stuff from people and run away with them, which would make the people pissed off. However, they weren't made to function like this.
Originally, the mouse droids were made to be household pets that had a little cage and a little food bowl and a water bottle, and their cage had to be cleaned regularly. However, all the mouse droids started making mechanical crap everywhere around the houses owned by the people who owned them, so they constantly got returned. Eventually, things started being a little bit lonely around the Death Star, so some of the stormtroopers asked Darth Vader for a pet. He said yes, and the stormtroopers bought all of the mouse droids. Now things started getting really noisy around the Death Star. However, the stormtroopers soon found out the mouse droids were extremely annoying, as the mouse droids stole all of their food. Everyone started thinking about giving them back, however soon everyone decided to make them carry messages. They also were once used to guide new stormtroopers around the Death Star, however they soon stopped doing that because they had a horrible sense of direction. Plus they were too fast. They also had another job as controlling the computer pointer, as people that worked on the Death Star could plug the mouse droids into the computer. This was their most popular use.