The Mouse is a minor enemy that first appears in the Gloomy Manor in Luigi's Mansion 2. Unlike the Mice from Luigi's Mansion, these mice are living, and not Ghosts. When Luigi shines his light on a mouse, the mouse quickly escapes from where Luigi is. If Luigi accidentally steps on a mouse he will loose 5HP.
Just like in real life, mice are found near mouse holes and will constantly keep reappearing. To defeat a mouse, you can either suck them up with your Poltergust 5000 or flash your Strobulb at them. If you decide to use your Strobulb on a mouse, they could drop a coin, a heart, or nothing at all.
Gold Mice are much rarer than regular mice and disappear after a few seconds, so you need to act fast if you want to catch one. If you do manage to catch one by using your Strobulb, you will be rewarded with a Gold Bar.
Mice also play a couple important roles in the game, such as in the begining a mouse is holding the key that Luigi needs to enter the Gloomy Manor. Another example is when a mouse in the Haunted Towers is holding one of the 13 Gemstones in the Seedling Laboratory.