To unlock creature costume skins you must complete a specific task which is to jack a certain number of a jackable creatures or kill a certain amount of the non-jackable kind, requirements for the skins can be found at the end of an episode when it shows you how much you have completed of a certain episode, but I will still provide a list of skins.
Battler: Jack 35 Battlers.
Bratgirl: Defeat 130 Brat Girls.
Doom Monkey: Defeat 500 Doom Monkeys.
EE-Lectric: Jack 50 EE-Lectrics.
Goar: Jack 30 Goars.
Halloween: Successfully complete 50% of the game.
Koo-ala: Defeat 150 Koo-Alas.
Magmadon: Jack 50 Magmadons.
Ratnician: Defeat 200 Ratnicians.
Rhinoroller: Jack 30 Rhinorollers.
Scorporilla: Jack 15 Scorporillas.
Shellephant: Jack 15 Shellephants.
Sludge: Jack 50 Sludges.
Snipe: Jack 50 Snipes.
Spike: Jack 50 Spikes.
Stench: Jack 50 Stenches.
Valentine: Collect all Gold Idols.
Voodoo Bunnie: Defeat 200 Voodoo Bunnies.
If you find this solution to be unhelpful, please tell me why in a comment and I will update it the next time I'm online.