Final answer:
The allusion to a 'Trojan horse' implies that the free makeover could have hidden negative consequences, just as the mythical Trojan horse was a gift concealing a threat.
Step-by-step explanation:
The meaning of the allusion in the sentence 'Hannah anticipated that the free makeover her friend was promising would turn out to be a Trojan horse' is that Hannah suspects the free makeover might have hidden intentions or negative consequences – similar to the Greek myth where Greeks gifted a wooden horse to the Trojans, which was received as a token of peace but contained Greek soldiers who later attacked Troy from within. This allusion suggests that what appears to be a generous offer might actually be a deceitful trick or carry an unforeseen backfire.