The Creature from the Ego is a bizarre invisible monster caught on Rialto IV. He's violent, brutal, bloodthirsty, and a sucker for insincere flattery. He can be found in Cell 105 of the brig on board the DeepShip 86.
Roger can hear him and sense his presence, but cannot see him. Which, according to the Narrator, is a good thing if one has just eaten. The creature does, however, visualise for a fraction of a second every now and then by running into the forcefield. He also does so when Roger talks to him and says something like "Hey, handsome! Let's see that pretty li'l face!".
The Narrator also noted that if Roger could get his arm through the force field (which he can't), the Creature from the Ego would nip it off at the shoulder and he would die horribly in a puddle of gore as the blood fountained from the gaping wound. Yet, he predicts that Roger's immediate thought would still be "Awesome! Let's see it!".
When the creature can be seen, he is described as someone who "could nip the macadamias off of a Labion Terror Beast in full-dervish!"
It would be pretty dumb to turn the forcefeild off. The creature would certainly devour Roger for dinner.
The Creature is a reference to the Monster from the Id from The Forbidden Planet. Ego and Id, along with the super-ego, are elements of human personality from Sigmund Freud.