Ruff, Underdog Champ is a rare Legendary Creature — Dog Soldier card from Mystery Booster Playtest Cards 2019 (CMB1), collector number 10. Current market price: $0.34, TCGplayer market prices via Scryfall.
All Hounds are Dogs. (We'll errata this to be true.) First strike, lifelink Underdog — If you've lost a game this match, Ruff, Underdog Champ and other Dogs you control get +1/+1.