I am well aware that the biology of seemingly Earth-like species on Oddworld is nothing like that which we are familiar with. But the "beetles" with skeletons, well, apart from clearly not being an arthropod, but something new, it is still recognisable as such. Arthropods and vertebrates both have hard parts of their body for muscle attachments and hinged limbs, and threading an endoskeleton through an exoskeleton, while somewhat redundant and raising interesting implications for the placement of skeletal musculature, is not all too much of an issue with some imaginative interal studies, though being such an unusual combination I don't think I'll be able to find any papers on the subject (unlike with the physics of Godzilla), but overall you can still end up with something recognisably coleopteran.
Octopuses, however, are a very different story. They are defined, above all else, as lacking any hard parts besides the ends of their beak. They motilate using muscular hydrostat system similar to that in our tongues. The modifications that would be made to an octopus to render any relation to the Glukkon or Gloktigi form make it stop being a mollusc. There is no possible justification for calling such a creature an octopus.
Similar issues arise when I compare Glukkons to Oktigi. Oktigi are closer to octopuses, their face and head is peculiar and seems based on a hard underlying structure, but it can be made workable as having an octopus decent. We might even be willing to use a similar model as the basis of the Oddworld variant of the octopus.
I short, I could probably make any one of the peculiar claims about the ancestry of Glukkons and Oktigi work in a biological and evolutionary sense. But all of them? That's a tall order. I need many more geological periods and orders of removal than could justify calling them "closely related". Birds with mammary glands are a cakewalk. Octopoda siring a vertebrate species? Trickier.
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