Sand Ripper is Four Arm's natural predator. Not every sentient species is at the top of the food chain. Most of these aliens have great weaknesses, so their natural predator would over power them by taking advantage of that. Captain Kork's species is naturally stronger and more vicious than the average Pisciss Volann, Tyrannopede is stronger than Vaxasaurians, there would be some kind of predator that's either faster or more reflective than a Kineceleran such as the fanmade Whiplash, Rock's fanmade predator has jaws strong enough to destroy his rock body, and Way Big is weak against this Virus concept that never came to fruition. A fan made version of this virus is simply a vicious small insect that infects the To'kustar with a virus. All these are really interesting concepts and I do partially agree when you say beings such as Petrosapiens and Ectonurites are too strong to have a predator, but I also disagree. Not all Ectonurites have their second skin makign them prone to light, right? And Anur Phaetos is the Galaxy's greatest mystery, meaning there are things that lie there we don't know about. Maybe even a light producing species that targets Ectonurites without their second skin. And Petrosapiens are weak against sonic vibrations, so maybe some kind of sonic screaming beast strikes them.