It is said that millions of years ago, when the world was first formed, all was shapeless, forming and re-forming. This time is called the Plasticene Era.
Primordial Ooze is a blob of this world-stuff, somehow left over. It can take on the basic shape of anything, but will retain its softness and bright colours. Thus it will only fool the traveller in near-darkness.
As a result, and because of their lack of defences, they mostly confine themselves to empty places in the underworld.