Horses Will Not Spawn at All

NOTE: This bug happened in Survival and Creative.

Whenever I start a NEW world in 1.6.2, I cannot find any horses or donkeys. I have searched everywhere, and have had no luck. I do know that they spawn only in the plains biomes on newly generated chunks. I have tried multiple worlds, both survival and creative, and have gotten nothing.

I have reason to believe it has to do with Mac OSX, but I could very well be wrong. I have played Minecraft 1.6.2 on a Windows 7 PC, and have found horses no problem.

I would very appreciate a resolution soon.

A forced crash report will be included in this post if need be