What part of a pig is filet?

There is no part of a pig called "filet".

"Filet" typically refers to filet mignon, which is a cut of meat from the tenderloin of a cow.

Pigs do not have a tenderloin in the same way that cows do.

However, pigs do have a cut of meat called the tenderloin, which is a long, thin muscle that runs along the backbone. It's often called pork tenderloin and is a popular choice for grilling or roasting.