Clone Codex

Clone Codex, storing Monsters
Clone Codex is a book in game that allows you to store monsters whose DNA you've captured. After going to incubate your Clone and bring it to life, you can choose to either equip it and use it in battles or add it to the Codex. Eventually this will give you a reward for completing an entire page or one line. Some clones can be ridden so this is also a reason to collect and add to your Codex. You can also check another player's info and see what they have collected. If they have collected one you want, you can gain some information about it by mousing over the icon you are interested in. Players cannot see you inspecting them. Mousing over the item gives you more information such as: Item Type, Clone Job, Clone Status, Current Level, Required Level, Revival Counts, if the Clone can be dyed and how much it normally sells to a vendor. You cannot see skills.

Synergy
When you finish a line in the Codex you will recieve Synergy. When you complete a line in the Clone Codex you will be able to use various Synergy buffs, located at the bottom of the page under the lines they come from. As you run out of Synergy Points you are able to register additional clones to replenish your points. This means you can add another clone to the Codex by incubating the DNA then adding it to the Codex.

Tips
You can also buy DNA or Clones from the Auction House, trade with other players or just farm them from the Monsters you are looking for. (If you're wanting a Poky then farm Poky, etc.)

However, the drop chance lowers the higher you are compared to the level of mob. (See: Catching a Clone/Pet for percents) 

So after a certain level it might be best to just trade with another player or check the Auction House for the DNA or Clone you're looking for. Alternatively, you could make a seperate character and farm mobs you weren't able to acquire on your main character. Comes in very handy if you are missing low level mobs.

Especially if you out level the mobs while you're farming.

For Additional Information about clones, the Codex and more try checking out Heva Clonia Clones