Although the aliens are never entirely seen, some among them are obviously of rather considerable size, with claws the size of Spiff. When Spiff mentions Scum Beings, he will usually accompany their name with epithets such as "hideous", "disgusting" or "dreaded".
Spiff also mentions being chased by Scum Beings from Planets Q-13 and Q-B in outer space. The spaceships appear disproportionate to the morphology of the scum beings, leading one to think that they exist in different sizes and shapes. In the first two scenarios, the scum beings represented Calvin's teacher, Miss Wormwood.
Once, Spiff is being chased by a ship much akin to Scum Being craft, and the occupants are seen. As they are not named, it is unknown whether or not they are Scum Beings.