Minecraft – Will horses spawn in old worlds

One work around is to run your own Minecraft server. The a dis-advantage this the Nether isn't currently working. You can launch the Minecraft server to load single player worlds. Just find your old ./world# folder in your .minecraft directory. When launching the server be sure your world matches the world name in the server.properties file.

I believe Mojang mentioned going this route in the future with Single player. Having the client launch a server instance locally and connect to it.

When this is fixed in future versions it should be a simple copy of the ./world folder back to your .minecraft directory and you will be able to load the world normally again.

No, ruins and villages are generated with the world, not actively built. If that changes in a future update, then it might not be required then, but for now all new geographic features require you to generate new chunks.

The usual way of doing this is just making a new world with all new chunks, but if you walk to places that you haven't yet, then new chunks will generate. (Potentially with new features.) It's still recommended that you generate a new world though since you might have to walk a while to get to new parts, the boundary between versions might not line up right, and biomes might shift(especially with the addition of a new one), but if you haven't explored much or really like it so far, it might be worth it.