The Deadly Mouse is an enemy in EarthBound that appears in the Fourside Sewers. It has very high guts, making the chance of critical hits being very high. It can also "bite using poisonous fangs", which can poison your party members. They often attack in groups, and can be very annoying to your party. Make sure every mouse is defeated before healing the poison, or you will just be wasting PP as the mice will probably just poison you again. Though not a serious threat, make sure to heal the poison if you don't want to keep running back and forth healing.
They are one of the few enemies that have different battle themes for different members of their family. The Rowdy Mouse uses "Battle Against a Weak Opponent" while the Deadly Mouse uses "Battle Against an Unsettling Opponent".