Rin and Seri, Inseparable
Oracle Text
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.
, : Rin and Seri, Inseparable deals damage to any target equal to the number of Dogs you control. You gain life equal to the number of Cats you control.
I like how it's kind of an alternate take on Fable of Wolf and Owl and I wonder how many playable Dogs I'll realize exist thanks to this one.
Honestly, the only opportunity to put "Cat Dog" instead of "Dog Cat" as the creature type, though I suppose it's to eliminate copyright.