No, it is not part of the unmodded game.
Mo Creatures is a modification (mod) for Minecraft. You can get this mod by downloading it from The Minecraft Forums.
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It depends what website you are downloading it from.
Depends on the mod. Either you mod folder in Run>%appdata%>.minecraft or they have to be installed into the minecrat.jar file itself. Check the mod details of whatever mod you're downloading.
Like all "Hijackers" you can get it by downloading something. Such as the herobrine mod for minecraft.
If you download Minecraft from the official web site then NO. The only time you can get one is from possibly downloading a mod or the application from an untrusted source.
No, thats impossible, the mod could only modify new worlds that you create after you install the mod.
you cant
Impossible.
you cant
Not without a mod.