Mabel, Heir to Cragflame is a rare Legendary Creature — Mouse Soldier card from Bloomburrow (BLB), collector number 224. Current market price: $0.35 (foil $0.40), TCGplayer market prices via Scryfall.
Other Mice you control get +1/+1. When Mabel enters, create Cragflame, a legendary colorless Equipment artifact token with "Equipped creature gets +1/+1 and has vigilance, trample, and haste" and equip {2}.