Rin and Seri, Inseparable is a mythic Legendary Creature — Dog Cat card from Secret Lair Drop (SLD), collector number 2668. Current market price: $14.83 (foil $14.78), TCGplayer market prices via Scryfall.
Whenever you cast a Dog spell, create a 1/1 green Cat creature token. Whenever you cast a Cat spell, create a 1/1 white Dog creature token. {R}{G}{W}, {T}: Rin and Seri deals damage to any target equal to the number of Dogs you control. You gain life equal to the number of Cats you control.