What if this monster is a generation 1? Maybe in gen 2 they wrote a code to make them not evolve into something horrid. Then they carried it onto Gen 3 giving them extra set of lungs and a stronger exoskeleton ETC. The point is maybe it is a gen 1 and didn’t write the code.
Don’t tell me it’s impossible, cause we don’t know what year it is there and they have healing technology, a device to bring people back from the dead, spaceships and more to come, don’t forget that the planet has extraordinary wildlife, monster and other created worlds