I've been exploring my world for a few days now, but I can't find any cows. I'm temporarily living in a village in the desert, but I frequently visit Tiaga, grassy, forested, and mountain biomes - tons of pigs and sheep, really tons of them, found some wolves, but no cows. I kill all the pigs I find, hoping the cows will spawn, but no luck so far. The wiki doesn't tell me which particular biomes are supposed to spawn cows, so I don't really know if I should seek out some other biome. (Come to think of it, I'm not sure I've even found chickens yet.)
I explored until I found a swamp biome, and found my first herd of cow. And I do mean cow. One. Dang, not much I can do with that! LOL
lol... go back later again and again.. despite what others have said, I've depopulated areas of certain mobs and when I've gone back later there's been more. The way I read the wiki, they'll only spawn in certain chunks but will continue to do so.