I dunno if this theory was suggested yet, but here goes.
Based on one of Cabot’ quotes where he wonders about how Ebonstar got a hold of equipment to study/experiment on monsters so quickly after they appeared, and figures out it couldn’t have been done, I’d assume that it is most likely that these monsters were created by Ebonstar from the indigenous wildlife of Shear.
Goliath - heavily altered Reaver/Sloth hybrid.
Kraken - heavily altered Mammoth Bird/Harpie hybrid.
Wraith - heavily altered Blitz Leopard/Marsh Strider hybrid.
Behemoth - heavily altered Sloth/Megamouth hybrid.
That would explain why Ebonstar had the equipment to experiment on them, why they’re so different yet have similar characteristics(3 stages), how they suddenly appeared on Shear when they don’t seem to have any technology(especially space travel) and why they so relentlessly attack and destroy human built facilities despite having plenty of wildlife for food.
It’s not a hard guess that such biological weapons would be very tempting for Ebonstar.