Raging Battle Mouse is a rare Creature — Mouse card from Wilds of Eldraine Promos (PWOE), collector number 143p. Current market price: $0.24 (foil $0.31), TCGplayer market prices via Scryfall.
The second spell you cast each turn costs {1} less to cast. Celebration — At the beginning of combat on your turn, if two or more nonland permanents entered the battlefield under your control this turn, target creature you control gets +1/+1 until end of turn.