Trojan Horse

The Trojan Horse was a hollow wooden statue of a horse used by Agamemnon and the Greek army to invade the city of Troy. The First Doctor did not, at the time, believe the Trojan Horse to be an actual historical event, but rather an invention of Homer for his account of the Trojan War. Threatened with execution by Agamemnon and Odysseus if he did not devise a method to swiftly defeat the Trojans, the Doctor first hoped to create flying machines for the Greeks. However, when Odysseus insisted the Doctor be the first test pilot, the Doctor reluctantly designed the Trojan Horse for the Greeks. (TV: The Myth Makers) The Meddling Monk later revealed this to Dodo, considering it an example of the Doctor's hypocrisy regarding the Laws of Time and non-interference. (AUDIO: The Outlaws)

Much later, the Fourth Doctor stated that he didn't come up with the idea of the Trojan Horse. (TV: Underworld)

As a metaphor
The story of the Trojan Horse was well-known, and was often used as a metaphor for something dangerous disguised as something harmless.

Arranging for Jackie Tyler to offer himself as a sacrifice to Tycho Fairbank as a ruse to thwart the Glubby Glub plot, the Ninth Doctor called her his "Trojan Horse", noting that he had met the original. (AUDIO: Retail Therapy)

Jack Harkness once described a bomb disguised as a scuplture as a "Trojan Horse". (PROSE: The Beauty of Our Weapons)

During the Last Great Time War, the War Council agreed that the technical division's postulation to retroactively insert a Trojan Horse program into dematerialisation circuits before they were appropriated by the Daleks would be an effective defense, though the possibility was recognised that the Daleks could discover such a program and turn it against the Time Lords, with a subcommittee being assigned to look into the ramifications of this course of action. (PROSE: Dalek Combat Training Manual)

The War Doctor compared the mining ship on Asteroid Theta 12 to the Trojan Horse. (AUDIO: A Thing of Guile)

Behind the scenes
Technically, the Doctor is accurate in denying the idea for the Trojan Horse was his. The Doctor's creation of the Trojan Horse is an ontological paradox; the Doctor did not come up with the idea, but knew of it from Homer, telling Steven Taylor that he thought it was fictional. However, Homer's account was based on the real Horse, which the Doctor designed for the Greeks. Thus, neither the Doctor nor Homer can truly be credited with the invention.