.... so I was looking through my models, and the Temple-guard helmets, sure, I can see what they are, but what exactly is Chakax' helmet made from? What kind of animal? It seems to be related to the Stegadon, but it's too wel develloped for a juvinile or infant Steg, and it sort of looks like some very old lizardmen models of the temple-guards, but how does that fit in present continuity?...
Interesting question. If you read the fluff of the item (helm of the prime guardian) it says it once belonged to the first TG ever, a very long time ago. I suppose it would be safe to assume it is the head of a creature that is now extinct, or even just one of the many varieties in the jungles of Lustria that GW have not described yet.
It may well have just been a very old stegadon, back from when they were much smaller. After all, we've seen them nearly double in size since their last model generation!