Mouse is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

How it works : Mouse

Original English text — Wizards of the Coast / Scryfall

No additional ruling has been published for this card.

Interactions with other cards

These cards' text mentions Mouse.

Might of the Meek

Target creature gains trample until end of turn. It also gets +1/+0 until end of turn if you control a Mouse. Draw a card.

Sword of the Squeak

Equipped creature gets +1/+1 for each creature you control with base power or toughness 1. Whenever a Hamster, Mouse, Rat, or Squirrel you control enters, you may attach this Equipment to that creature. Equip {2}

Lupinflower Village

{T}: Add {C}. {T}: Add {W}. Spend this mana only to cast a creature spell. {1}{W}, {T}, Sacrifice this land: Look at the top six cards of your library. You may reveal a Bat, Bird, Mouse, or Rabbit card from among them and put it into your hand. Put the rest on the bottom of your library in a random order.

Rockface Village

{T}: Add {C}. {T}: Add {R}. Spend this mana only to cast a creature spell. {R}, {T}: Target Lizard, Mouse, Otter, or Raccoon you control gets +1/+0 and gains haste until end of turn. Activate only as a sorcery.

Three Blind Mice

(As this Saga enters and after your draw step, add a lore counter. Sacrifice after IV.) I — Create a 1/1 white Mouse creature token. II, III — Create a token that's a copy of target token you control. IV — Creatures you control get +1/+1 and gain vigilance until end of turn.

Valley Flamecaller

If a Lizard, Mouse, Otter, or Raccoon you control would deal damage to a permanent or player, it deals that much damage plus 1 instead.

Manifold Mouse

Offspring {2} (You may pay an additional {2} as you cast this spell. If you do, when this creature enters, create a 1/1 token copy of it.) At the beginning of combat on your turn, target Mouse you control gains your choice of double strike or trample until end of turn.

Enchanted Carriage

When this Vehicle enters, create two 1/1 white Mouse creature tokens. Crew 2 (Tap any number of creatures you control with total power 2 or more: This Vehicle becomes an artifact creature until end of turn.)

Recruit Instructor

Whenever Recruit Instructor attacks, draft a card from Recruit Instructor's spellbook. Valiant — Whenever Recruit Instructor becomes the target of a spell or ability you control for the first time each turn, create a 1/1 white Mouse creature token.