This is from the heart of a mammal; it does not make a difference what mammal.
You probably need some clues here for orientation. The bottom two-thirds of the photograph is composed of the ventricular cavity (right side). The right hand border, or the 3 o'clock border near the ruler, is the cranial border of the right ventricular cavity. The large vessel pointing toward 1 o'clock is the aorta. It is not involved in this process.
This is sort of a hide and seek case. A review of heart anatomy might be in order if you don't see the lesion.
This young animal likely died of left heart failure, and it was young; there is a congenital lesion here.
The major artery portrayed is the pulmonary artery. At its base is the origin of a coronary artery.
