For beginners:Download Mo’ Creatures Mod(Animals Mod) :
To download you need Forge 1.6.4 you can get it from HERE and Custom Mob Spawner click HERE to download.
When your downloads are done go to the start menu and type in run then type in %appdata% like picture below.
When you open run open .Minecraft folder. Now if you have a mods folder skip the orange part.
If you dont have a mods folder create new folder and name it "mods" without quotes.
when you create the folder put both Mo’ Creatures Mod and Custom Mob Spawner in the mods folder.
After all launch Minecraft with Forge profile. Make sure its 1.6.4
For more advanced people: