Mattresses are large, friendly, pocket-sprung creatures which live quiet private lives in the marshes of Squornshellous Zeta. Many of them get caught, slaughtered, dried out, shipped out and slept on. None of them seem to mind and all of them are called Zem.
Mattresses have a rather unique vocabulary befitting their rather unique physiology, and it is recommended to pick up a copy of Squornshellous Swamptalk before visiting them (or The Ultra-Complete Maximegalon Dictionary of Every Language Ever, if one is a glutton for punishment, although this one omits the word "floopily").
On rare occasions, Mattresses will rear their oblong bodies, heave themselves up into the air and remain quivering there for a few seconds to look through the mist at something in the distance. This takes every bit of their strength so they rarely bother to do it.