Rin and Seri, Inseparabler is a rare Legendary Creature — Dog Cat Scientist card from Mystery Booster 2 (MB2), collector number 601. Current market price: $0.42, TCGplayer market prices via Scryfall.
All Cats you control are Dogs in addition to their other types. All Dogs you control are Cats in addition to their other types. Cats and Dogs you control have haste.