In computing, a Trojan horse or just a Trojan is malware that appears to do something good, but usually illegally steals data from your computer (often secretly) for profit.

The legend is that they took it inside their city, not knowing there were Greek warriors inside it, and that night they opened the gates to let in the Greek armies. Since then 'trojan horse' is an expression for sneaking in a disruptive element, as in computing.

yes there was a Trojan Horse.

A Trojan Horse.

The Trojan horse was pulled into Troy, hence 'Trojan'.