What is the name of the taco filled with brain (cow or pig)?

The taco filled with brain from a cow or pig is called tacos de sesos.

Sesos is the Spanish culinary term for animal brains, especially those from cattle or pigs. In Mexican cooking, the brains are cleaned, gently cooked, seasoned, and placed in a tortilla, often with onion, cilantro, salsa, or other taco garnishes.

Tacos de sesos belong to the broad tradition of Mexican tacos made with offal, known as casquería. That tradition also includes tacos de lengua, made with tongue, and tacos de mollejas, made with sweetbreads. These names identify different organs, so they are not interchangeable.

The term sesos is plural because it literally refers to brains. Brain dishes also appear in other cuisines, including French preparations such as cervelle de veau. The taco name describes the filling rather than a particular cooking method, so recipes can vary between boiling, steaming, frying, or seasoning the brains before serving.