The canonical appearance of the xel'naga are squid-like creatures (similar to the void thrashers, who are implied to be related) that would easily fit among the zerg armies. (The same applies to the hybrids.)
Prior to the canonical depiction, xel'naga were often imagined by fans as looking identical to the protoss. I have no idea why. Indeed, the Dark Voice looked like a protoss (or a mouthless hybrid?) before it was given the squid form.
What other ways do you think the xel'naga could be drawn to make them look distinct?
I have a few suggestions to start with:
- They looked really weird and abstract, but in a way that suggests the protoss modeled their clothing after them.
- The xel'naga were actually tiny squids which wore the sarcophagi and robots from SC2 as mobile suits, similar to the Daleks from Doctor Who.
- The xel'naga looked like adorable, cuddly stuffed animals with creepy, soulless eyes.
- Like the elder things or flying polyps from the work of H.P. Lovecraft.
- They looked like surreal entities that would not be out of place as the villains of a Japanese scifi or fantasy anime. Things like anatomy resembling sculpture rather than flesh, multiple arms held in poses reminiscent of Hindu deities, a lack of a distinct head, lifeless masks instead of actual faces, numerous eyes in strange places, bodies composed of platonic polyhedrons, immense sizes, each one having a unique form, holding meetings while floating in gigantic voids, etc.
- They resemble the various sphinxes, angels and so forth of ancient Middle Eastern and Renaissance art. These can get very weird, like having multiple heads from different animals, feathered wings, scorpion tails, wheels covered in eyes, lion-headed snakes made of fire and other weird surreal patchwork anatomy.
- They resemble demons from Judeo-Christian mythology, particularly the surreal and horrifying artwork of Hieronymus Bosch.
- Like the Vorlons from Babylon 5, their appearance varies depending on who is looking at them. They have no true form.
- Their forms are so grotesque and horrifying that anyone who looks at them goes insane, turns to stone, dies, their eyeballs explode, or something similarly horrible. Even recordings have the same effect. Thus, nobody knows what they actually looked like.