My wife wants to find a horse, but the only plains on her map is about 80x80 squares, and no horses are spawning. Do they need a certain amount of space to spawn? She doesn't want to start from scratc, since she's invested a lot of time into her current world.
It may be that the biomes on her seed have shifted with the update and what looks like a plain is no longer a plain (weather-wise and spawning-wise). You can check this by creating a new world using the seed string for her old on and comparing what biome is located in that spot (using the map coordinates). You may also find that was doesn't look like a plains biome on her old map may be a plains biome on the new version of it). In that case, it may be a matter of placing down grass blocks in the old world where the new world has a plains biome (if, for example), that area is a desert in the old world).
If she has a lot of time invested in her world... she may have surpassed the spawn cap for passive MOBs in her fared animals alone... so some of these may need to be thinned out in order for new spawns to start up again.
To add to what UpUp said...although a biome shift is not likely unless the world is actually older than the last time when new biomes were added (ie. when Jungles were added)... it may be worthwhile just to check a new world with the same seed (in Creative Mode) just to see if horses can spawn in that area.
While looking, kill off every passive mob that you spot, as they'll get in the way of horse spawning.
Stay fluffy~
To add to what UpUp said...although a biome shift is not likely unless the world is actually older than the last time when new biomes were added (ie. when Jungles were added)... it may be worthwhile just to check a new world with the same seed (in Creative Mode) just to see if horses can spawn in that area.