These same rules apply to Unstable Shapeshifter (hence the funny quads with shifter/immortal/hognose/charger)
Relevant code is `transform` in game.rs
transform also applies mutation effects when instance has mutant status, so casting jelly on a mutant (note spider cows are not mutants) creates a jelly with a mutant active (but cast/attack/hp are overridden by jelly spell after transform)
If spider cows are not mutants, then why does casting jelly on a cow grant it a random active?
Additionaly, does the mutant trait have any other interactions? There aren't any cards that directly state effects on this trait.