I was never sure why Cain had a herd or flock when humans were supposedly vegetarian until after the flood. I suppose he could have kept them for their fur or milk.
It seems to me the idea of Cain and Able making sacrifices to God was added later once this practice became customary among pagan nations in the same manner as the spinning sword at the entrance to the Garden of Eden was added to the Bible even though swords and the ability to mine and forge metal came along much much later in human history.