So we got normal huge dinos, but why not a few steps further and have super rare and ultra rare editions of these...

In the example, I'm gonna use a dodo.
Dodo (guessing spawn rate is 1/100) spawns every so often, and from that dodo, there is another 1/100 chance of a giant dodo. and from thata giant dodo, a 1/100 to spawn a beheometh dodo.

These would obviously be larger, and higher leveled.

Average for Large is (high primitive)ramshackle - journeyman
Average for Giant would be apprentice-mastercrafted
Average for Beheometh would be journeyman - ascendant

In terms of chances, a beheometh dodo would have around 1/10000 chance to spawn (which is the reasoning I say to give it access to Ascendant tools)

And as well as size scaling, once you figure it out, even particle effects and unique coloring as the size goes up. (obviously special abilities too :p)