Air elementals are creatures that upgrade if the hero has the Ranged Attack skill, but do not upgrade if they have Melee Attack.
For the same reasons, Gremlins can be better than Master Gremlins.
Also, Green/Red dragons can turn out to be stronger than Gold/Black dragons if there is the Resurrection magic... :rolleyes: but this is, of course, a rare occurrence.
In some situations, it is not beneficial to upgrade skeletons, as they will be restored to 2/3 of the number they would have been if they were basic skeletons.
You can also add Psionic creatures due to their positive magic, as well as Gogs. But overall, this is all too theoretical. In general, Warriors are the ones that improve. :)
