Metaphors similar to "Trojan Horse"

Solution 1:

A Wolf in sheep's clothing metaphor may also fit the context.

is an idiom of Biblical origin. It is used of those playing a role contrary to their real character, with whom contact is dangerous. As a fable it has been falsely credited to Aesop and the theme is now numbered 451 in the Perry Index. The confusion has arisen from the similarity of the theme with fables of Aesop concerning wolves that are mistakenly trusted by shepherds; the moral drawn from these is that one's basic nature eventually betrays itself.

Source:Wikipedia

Solution 2:

Fifth column (Wikipedia) comes to mind.

A fifth column is any group of people who undermine a larger group—such as a nation or a besieged city—from within. The activities of a fifth column can be overt or clandestine. Forces gathered in secret can mobilize openly to assist an external attack. This term is also extended to organized actions by military personnel. Clandestine fifth column activities can involve acts of sabotage, disinformation, or espionage executed within defense lines by secret sympathizers with an external force.

Patrick M added

Critically, however, the Fifth Column idiom originates from supporters or sympathizers for the attackers converted or persuaded to change sides from within the defenders. Almost all usages of Trojan Horse imply an attack originating from outside under disguise.

Emilio Mola, a Nationalist General during the Spanish Civil War, told a journalist in 1936 that as his four columns of troops approached Madrid, a "fifth column" of supporters inside the city would support him and undermine the Republican government from within.

Note also that a column is specifically a military formation, making General Mola's initial metaphor slightly off. These may give the wrong impression of your metaphor to a historically minded or pedantic audience. Nevertheless, Fifth Column has spread widely in usage since its coinage and matches your example meaning of undermine or destroy from within very well.

more synonyms (thesaurus.com)

Solution 3:

I would disagree with the definition given in the question. A Trojan Horse is specifically a gift, something given under the guise of friendship intended to destroy your enemy.

Therefore, you could say a poisoned gift or a poison apple.