Dissaperance of cows from the world

When I first created the world, there used to a few cows around. Now, there are no cows to be seen, only sheeps, pigs and chickens in mass groups together. As I keep exploring the environment, I still could not find any cows of the sort.This is strange as I was sure there were cows around as when I first spawned, there were a few cows nearby me. Now, it seems that have vanished. Even the spawns consists of nothing but sheep, pigs and chickens.