Known by many names, but commonly referred to as "Proteans" by humans (and their closest allies), members of this alien species can resemble any other species of creature or plant. The resemblance is superficial, though, because there are always telltale signs that a Protean stands before you.
The first is that all Proteans of a given species appearance will look...blurry, for lack of a better term. All of the features are present, but fine details are lacking. Furthermore, any Protean of a given species appearance will look like one of @100 different, specific appearances (not counting color, attire, or carried/worn equipment) per gender (if the species has such) as if the form were cloned or otherwise chosen from a limited menu of options. For that reason, those variations will typically be called (again, among humans) "avatars" with a numerical designation. So a human-like Protean calling itself "Steve" will also have a "surname" like Male Avatar #47.
Also, within a given "avatar", all Proteans sound exactly the same. So Steve Male #47's voice will sound identical to Suzy Female #81's.
Proteans also appear to violate the known laws of physics. Proteans seem to require no nutrition. They consume no food or water, do not appear to photosynthesize, or take in sustinemce in any known way.
Furthermore, not only do Protean bodies look unfinished, they vary in size and mass from moment to moment in two ways. In the more stable or "resting" state, this oscillation is as regular as a pulse, and is identical to all Proteans regardless of species form or avatar. This oscillation has been measured at 93,000hz, so some other species can actually hear this. There is no indication as to whether Proteans are aware of this.
On top of that, Proteans can increase or decrease their physical dimensions by up to 50%, with all the expected attendant fluctuations of mass. No one has been able to determine where this mass is going to or coming from. This is all the more puzzling since, as noted, they do not seem to consume food or other energy sources to survive and grow.
One human researcher- Academician Ichiro MacDougal-Rosenstein III- had a theory as to Proteans' true nature, based on Edwin Abbott Abbott's Flatland. He posited that Proteans were not of our dimension, but were instead aliens from a higher dimension protruding down into ours. Like a cone passing through a plane of Flatlanders would appear as a circle or oval of various sizes to them, depending on its angle and depth of protrusion, Proteans vary in size depending on how much of the postulated alien is intruding into our dimension, and how. Any energy and mass required to do this is being supplied in the higher dimension.
The MacDougal-Rosenstein "Flatlander" Theory would also explain the last way in which Proteans break the laws of physics: Proteans seem to be able to teleport. Despite having no known technology to do so, Proteans can teleport- not quite intantaneously, and with obvious concentration and effort- over great and even interplanetary distances. As he put it, "It is as if you removed a stylus from a screen and simply touched it elsewhere on the same screen." Thus, Proteans have no known spacecraft...or homeworld, for that matter.