With Forerunner of the Empire already on the battlefield, whenever a Polyraptor you control enters the battlefield Forerunner’s “Whenever a Dinosaur you control enters” ability will trigger and can deal 1 damage to each creature. That damage will trigger Polyraptor’s Enrage (it triggers “Whenever this creature is dealt damage”), creating a token that is a copy of Polyraptor. The token entering will itself trigger Forerunner, which can deal 1 damage to each creature again, causing every Polyraptor on the battlefield to get dealt damage and each to trigger its Enrage. The interaction can create a cascading series of triggers and token copies. The relevant rules are that creatures entering the battlefield are put onto the battlefield when their spell resolves , copies copy the copiable values of the object being copied , and the copy rules about changing what a permanent copies don’t re-trigger ETB/leave events . Also note the official ruling that if multiple sources deal damage to the same creature at the same time, Enrage triggers only once for that creature (Ruling [2018-01-19]).
Step 1
Forerunner of the Empire is on the battlefield under your control and Polyraptor enters the battlefield (a creature spell resolves and is put onto the battlefield ).
Step 2
Forerunner’s triggered ability (“Whenever a Dinosaur you control enters, you may have this creature deal 1 damage to each creature”) triggers as a result of Polyraptor entering and is put on the stack.
Step 3
When that Forerunner trigger resolves, it deals 1 damage to each creature simultaneously. That damage event dealt to Polyraptor causes Polyraptor’s Enrage ability (“Whenever this creature is dealt damage, create a token that’s a copy of this creature”) to trigger.
Step 4
When the Polyraptor Enrage trigger resolves it creates a token that’s a copy of Polyraptor. The token is put onto the battlefield as a permanent (see ). The token is a copy and therefore has Polyraptor’s copiable characteristics, including Enrage ().
Step 5
The token entering the battlefield causes Forerunner’s Dinosaur-entrance triggered ability to trigger again. That trigger may resolve and deal 1 damage to each creature again—this includes every Polyraptor and token on the battlefield.
Step 6
Each distinct Polyraptor that is dealt damage by Forerunner’s effect will trigger its Enrage ability once for that damage event (and simultaneous multiple sources dealing damage to the same Polyraptor would cause Enrage to trigger only once for that creature per the ruling [2018-01-19]).
Step 7
Resolving those Enrage triggers creates more Polyraptor tokens, which enter and cause Forerunner to trigger again, and the cycle continues as long as triggers resolve and produce new entries.
Resolution
If you control Forerunner of the Empire and a Polyraptor enters the battlefield under your control, Forerunner’s triggered ability (the Dinosaur-entrance trigger) goes on the stack and may later resolve to deal 1 damage to each creature. Each Polyraptor that is dealt damage this way will trigger its Enrage ability once (Ruling [2018-01-19]). When each Enrage trigger resolves it creates a token that is a copy of Polyraptor; that token enters the battlefield (see ) and is a copy with the same copiable values, including Enrage (see ). Each of those token entries will cause Forerunner’s Dinosaur-entrance trigger to trigger again, which can deal 1 damage to each creature again and cause every Polyraptor on the battlefield to trigger Enrage again. This produces a chain of triggers and token creation; each instance of a Polyraptor being dealt damage triggers Enrage once, and each token enters as a copy of Polyraptor (see , ).
