There are no real details revealed. I always presumed that both Gurid and Margalard (his Orangutan equivalent) were beasts created by Sucellus during the godswar. As he had no intention of getting involved in the conflict they were probably mostly defensive weapons to guard his borders. He may never have bothered making a beat as great as a dragon.
On the other hand, it is quite possible that the great apes were creations for mortal mages from the Age of Magic. "Giant Animals" are listed in the Bestiary of Erebus under "Creations of Men: Perversions of Life." Most of these were made by Life mages of the school led by Leucetios, but creations were mostly hybrids of various creatures rather than one clear type of animal that is just over-sized. The Giant Great Apes could be the work of Kezef, the master of body magic whose experiments killed all the animals form the Grigi plains. It could be the experiment involved making a few of the animals so large and powerful that they killed off the rest, either directly or by ruining the food source.
