Rin and Seri, Inseparable

The joke is that the tribes are the two most beloved and least supported creature types in the game, and the design makes that scarcity the whole point. Dogs and Cats had spent decades as flavor labels on cards nobody was building around, so a four-mana legend that rewards casting them had to bring its own payoff or it would be an investment in an empty theme. It solves this by generating the opposite tribe when you cast one: every Dog spell makes a Cat, every Cat spell makes a Dog, so the board fills itself without needing a deep well of tribal cards. That self-seeding is what makes the activated ability worth building toward, since it scales off both counts at once (damage from Dogs, life from Cats), and each token you make from casting one side quietly pads the count on the other. The result plays as a token engine wearing tribal clothing: you are not really assembling a Dog deck or a Cat deck so much as an everything-adjacent go-wide deck that happens to trigger twice as often as a single-tribe build would. It is also one of the rare designs that treats two flavor-only creature types as a single mechanical unit, and the reason it works is that neither tribe had to be good on its own for the pair to function together.