Khanz, it is official OWI information that Glukkons evolved from squid-like creatures. Sorry.
I do believe that Native Glukks were plains dwellers, however. Just as Mudokons evolved from Birds, but native mudokons are not airborne, so Glukkons evolved from octopi, but native glukks are terrestrial. Just my theory, though.
As for why they turned bad, I agree that it was the Khanzumerz. As I explained in a different topic, Khanzumerz are the closest thing to pure evil on Oddworld (present company excepted, of course
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