Why aren't cows and pigs spawning in minecraft project edition?

I had a few to begin with. Then I had two left. I killed one by mistake, and the other one is now fenced up. I have lots of chickens and sheep, since I'm breeding them.

Is it because I explored the whole island? I hate restarting. I just found diamonds and gold so me don't want to restart.

Added (1). Nevermind up there. I restarted and found cows, but how can I get cows to start spawn g in my old world

Added (2). I meant up there minecraft pocket edition.
And Emily, I wrote that, I'm not on PC. DO you EVEN READ

You can type in /gamemode 1 (Which is creative mode) then you can get spawn eggs.