The animals no longer despawn and their spawn is based on the biome seed. In other words, the randomness of spawning was removed. There are several mods that relied on the prior mob spawning method to add custom mobs to the game, like Mo’Creatures, More Creeps and Weirdos, Humans+ and many more.

Installation:

Just download the .zip file and put it into the /.minecraf/mods/ folder


Download: Custom Mob Spawner Mod for Minecraft 1.5.2
You can also see all awesome Minecraft 1.5.2 Mods here: Minecraft 1.5.2 Mods
You can also see all awesome Minecraft Mods here: Minecraft Mods