Peasants and Invaders
The reason for many meats having a different name to the animal from which they are derived is one of early English culture.
During the 11th Century, after the Norman invasion, there were Anglo-Saxon peasants who would eat whatever they could - vegetables, berries - and would have to tend animals for the Norman invaders to eat.
The peasants would tend a pig (germanic word) and the Normans would eat pork (porc). The peasants would tend a sheep, but the Norman aristocrats would eat mutton (mouton).
Simple, when you know how!