Panharmonic Elemental

Mutate {2}{g}{u} (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, until the end of
your next turn, if a permanent entering the battlefield causes a triggered ability of a
permanent you control to trigger, that ability triggers an additional time.