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I want to cast Polymorphous Rush on a strong creature on my battlefield so that my Hamletback Goliath's ability will trigger. The creature that will become the copy is already on the battlefield, but when it becomes a copy of the strong creature, will this trigger my Hamletback Goliath's ability?
For reference, Hamletback Goliath reads:
Whenever another creature enters the battlefield, you may put X +1/+1 counters on Hamletback Goliath, where X is that creature's power.