Souvenir Snatcher

Mutate was built to stack abilities onto a single body, letting a pile of creatures share one growing tower of triggers. This Bird bends that mechanic into a theft engine by hanging its payoff on the act of mutating itself rather than on attack or combat damage: whenever it mutates, you gain control of a target noncreature artifact. Cast it hard for its five-mana body and it is a serviceable 4/4 flier with the trigger dormant, waiting for a future mutate spell; pay the mutate cost and you convert a mana premium into control-magic, pulling a mana rock, an Equipment, anything noncreature-and-artifact away from an opponent for good. The theft is permanent: there is no duration clause, so the artifact does not go home if the mutated pile dies later, and destroying the Bird after the trigger resolves changes nothing. What shapes the design is that mutate wants you to keep building on the same tower, and this rewards exactly that; each additional mutate spell cast onto the pile the Bird belongs to fires the trigger again, so a player holding several mutate pieces can strip artifacts off the table one resolution at a time. The narrow window is the only brake: you steal when a mutation resolves, not from passive presence. It is blue's control-magic tradition rerouted through a creature-growth keyword, the on-mutate hook doing the theft work that older permanent-steal effects did off the stack.