Regal Leosaur

Mutate {1}{R/W}{R/W} (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Whenever this creature mutates, other creatures you control get +2/+1 until end of turn.

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Ikoria: Lair of Behemoths

Regal Leosaur’s last ability affects only other creatures you control at the time it resolves. Creatures you begin to control later in the turn won’t get +2/+1.