Who warned the Trojans not to take the wooden horse?

The Trojans were not warned of the horse. In fact, they were deceived into believing the horse to be a gift from the defeated Greeks. Sinon, the deceiver, pretended to be angry. He claimed his fellow Greeks had deserted him when they left Troy. In fact, the Greeks had merely pretended to leave Troy in an attempt to trick the Trojans into a sense of false security. The plan worked. After the Trojans brought the horse within the walls of the city, the Greeks needed only to wait for the cover of darkness to attack from within the city.