What Happened:
Went to a new chunk to look for horses, but no animals were spawned at all. I could not find a single animal of any kind. Tried a few different new places, but nothing.

What Should Have Happened:
Should have seen at least some new mob spawns (cows, pigs, etc.)

Other Info:
The map was created a long time ago. There was an upgrade that added a lot of new features (strongholds, etc.), so we created a new one then and we have been using that one ever since. Aggressive mobs still seem to spawn at night and in darkness without problems.