Regal Leosaur
Creature — Dinosaur Cat
Mutate 
(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.
Tomasz Jedruszek
Magic Online Promos (PRM)
• #80967Rulings
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.
(wotc • 2020-04-17)
Rulings
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.
(wotc • 2020-04-17)