You will find mini mob embryos in dungeons and temples. The embryos can be placed into an incubator to begin a gestation process.

You will need to craft mini mob pellets to put the developed embryos into them, then you can throw a pellet to the ground to spawn a mini mob!

Mini Mobs can be of the following types:

Pig: will tackle the enemy.
Zombie: Smites your enemies, has strength and health bonus, and can pick up items to give them to you.
Skeleton: Ranged attack.
Creeper: Does not explode (thankfully), instead dashes at the enemy. From level 7 on there is a chance it will set the enemy on fire.
Spider: Can climb walls. From level 6 on, it has a chance to poison the enemy.
Soldier: It's a player mini mob, given a valid player name, it takes the corresponding skin.
Penguin: Fast on ice, snow, water, walks like a penguin.

The mini mobs gain experience with every attack they perform. With experience they can level up and increase their stats:

Items and blocks on this mod.

MM Embryo. Fin them in dungeons and temples.
MM Pellet. To put the mobs in.
MM Incubator. You ned RF Power to use it. Put the embrios and pellets in there.
MM Creative energy cell. Only available in creative mode to provide power and test the incubator.
MM Naming Station. Allows you to name items using dyes. Yes you can get color names.

For versions 1.2.21 and below CoFH Core Needed to get power:

Iskall Mini Mobs