In this short blog post I will explain how to get these mobs into the game.
These mobs are:
Giant
Green Villager
Skeletal Horse
Zombie Horse
Giant Slime
Giant Magma Cube

Normally you can't get these mobs without hacking or modding, but now in minecraft 1.7.2
you can use the summon command to spawn them.

Giant - to summon the giant you need to input the command:/summon Giant
The Giant is basically a giant zombie and most of the time it can one-hit you.

Green Villager - to get this particular Villager the game has to spawn one in the village with the profession of 5, but
the game doesn't do that anymore that's why it can never spawn normally.
To spawn this use this command:/summon Villager ~ ~ ~ {profession:5}

Skeletal Horse - This horse cannot spawn normally so it doesn't spawn within the game.
The command for this is:/summon EntityHorse ~ ~ ~ {Type:4,Tame:1}
You need to add the Tame extension because there is no way to tame it yet.

Zombie Horse - This horse also cannot spawn normally.
You'll also need to add in the Tame extension because there is no way to tame it yet.
command:/summon EntityHorse ~ ~ ~ {Type:3,Tame:1}

Slime - although it is not considered to be lost it's sizes are.
command:/summon Slime ~ ~ ~ {Size:#}
Replace the # with a number.

Magma Cube - same with the slime's desription.
command:/summon LavaSlime ~ ~ ~ {Size:#}
Replace the # with a number.