These creatures have, thankfully, never been seen in Teleleli. They are only known there through descriptions given by refugees from Barsoom, who say that they came into conflict with them in the distant past.
The original appearance of the creatures is not known, although they may have resembled Barsoomians, or even been a branch of that species. The prevailing philosophy of their society seemed to be based on a belief that the mind was separate to the body, even to the brain, and a hatred of the body combined with an idealisation of the mind. Possessing advanced science, they were able to alter their bodies. Perhaps this led to a vicious cycle where they found themselves in bodies instinctively alien to them, which caused their hatred of the body to grow, and led in turn to more extreme modifications.
Whether this is the reason, it is undisputed that they altered their bodies more and more. At one period in Barsoomian history they are reported as having soft, oversized, but otherwise human- or Barsoomian-like heads, but lacking any other body. Instead, they grew headless bodies, on which they 'rode', controlling them by unknown means.
After many centuries their bodies were huge, and resembled machines more than biological creatures, having a metal-like skin and weapons of great power, including poisonous black smoke and a 'Tenth Ray', which is described as a special form of light which incinerated whatever it fell upon. They are said to have had many tentacles instead of arms and legs, and to have been able to travel land and sea with equal ease, though not to fly (the name Sky-Tyrants derives from the fact that they towered over the Barsoomians).
It strikes me as a peculiar coincidence that the loa of Haitian voodoo
are said to 'ride' the worshippers whom they possess, and perhaps this
provides some clue as to the Sky-Tyrants' eventual fate. It may be that they managed to become altogether incorporeal. In any case, after a destructive war they are heard of no more in the history of Barsoom.