Definitions of Trojan horse:

• noun:   a large hollow wooden figure of a horse (filled with Greek soldiers) left by the Greeks outside Troy during the Trojan War
• noun:   a program that appears desirable but actually contains something harmful

  Example: "When he downloaded the free game it turned out to be a trojan horse"

• noun:   a subversive group that supports the enemy and engages in espionage or sabotage; an enemy in your midst