World Existing Before 1.6 Won't Spawn Horses

In my survival single player world it seems like I am experiencing some horse spawning issues (This map was created in 1.2.5). I loaded new chunks on this map and searched a few plains biomes, but couldn't find any horses.

Other mobs generate in my world that have been added since 1.2.5 (Like, wither skeletons). I don't know if this is a bug or a problem, or if I don't understand the horse spawning mechanics.