Guys, you might want to check out this video, which shows -- pretty definitively, if you ask me -- that horses will spawn randomly in old, pre-1.6 chunks:
Private Mod Note
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Village Mechanics: A not-so-brief guide - Update for 1.4! Added subsection on iron farming to the iron golems section. Only ten villagers required to spawn golems (down from sixteen)! Put to rest the "facing doors cancel each other" myth!