The Grimoire of Gaia mod adds in a massive variety of mobs. All of these mobs have great models and also have unique abilities and strengths. There are also unique weapons and even shops where you can buy different items.
How to install:
Other Versions:
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For Minecraft 1.5.2
https://minecraftlucky.com/dowload/file-473
For Minecraft 1.5.1
For 1.4.7/1.4.6
For Minecraft 1.4.5
For Minecraft 1.6.4