You unlock creatures by winning Scratch Cards and getting 3 out of the 6 slots being a creature.
You can check how many creatures you have and how many you have left by going to the Creatures section - Far left of the main screen of painting.
You unlock the Scratch cards by getting a certain amount of Lums in each level - most of the time this amount is 450
Once you have unlocked one you will be able to scratch it by pressing Y and bring up the correct sub-menu.
The scratch cards are all set and not random and you will not unlock the last creature until the last few Scratch cards overall so you will have to complete nearly every level inclduing Back to Origins to get this.
This is obviously help you towards:
I hope this helps!