When used in a dungeon, the Exit mouse will wave at the exit and the party will be warped there. The Exit mouse can only be obtained by speaking to the mother located in certain locations.
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In-game description
EarthBound
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If you use this in a dungeon or some such place, the mouse brings you back to the place where you entered. It's a very smart mouse.
Artwork of an Exit Mouse from the Mother 2 instructions booklet
Artwork of an Exit Mouse from the Mother 2 strategy guide
Trivia
Although normally Exit Mice cannot be used to exit the Cave of the Past, a programming glitch in Mother 1+2 allows Ness to use it and successfully escape in the final room where Giygas resides. After doing this, he appears at the entrance to the dungeon in which the Exit Mouse was obtained.
In the EarthBound Player's Guide, the section describing the Exit Mouse contains a screenshot displaying a text glitch. This does not occur in the final game.