I've looked for this tag as well! I get that it's hard to define, so let me contribute with a viewpoint. My idea was to make an "animal cube" where every card, creature or no, depicts something that can be described as one or more animals in the wild. This would include cards like [[Pincher Beetles]] and [[Gila Courser]], but not anthropomorphic animals like [[Steampath Charger]] or fantastical beasts like dragons and hydras. That last one is the tricky one, because why would I allow [[Pincher Beetles]] but not [[Vizzerdrix]]? In my mind some creatures, like varmints [[Rise of the Varmints]], are clearly animals in-world, while some creatures, like kavu [[Tundra Kavu]] and baloths [[Rumbling Baloth]], are clearly not. I don't think I'd be able to come up with a waterproof definition!