Horses not spawned?

I've updated my Minecraft client to version 1.6.2, with an already-progressed world in my files. I expected to play this, expand everything, etc.

However, upon spending hours upon hours of travelling between plains biomes, I had yet to see NO horses whatsoever. I have searched around every single plains biome I've been in, to no avail.

I made a new world, and I found horses within 10 minutes. I don't want to start a new world just to have horses in my game. Please help me. Thank you.