Fig. 2: Nupperibo
But, of course, they don't. Nupperibos are hunted down, herded up, and forcibly shaped into true baatezu form: the lemure (fig. 3). Lemures are actually one peg lower than the nupperibos, but are the only form that allows further advancement in the recognized baatezu ranks. Both nupperibos and lemures are used as expendable footsoldiers in the Blood War, and very few live to see a different fate.
Those few that survive (approximately one in a million) are then promoted to a first higher form: that of a spinagon (fig. 4). These devils are spies and messengers of the baatezu, and some of them can be quite cunning (a concept entirely alien to the mindless nupperibos and lemures). Now, as spinagons, these baatezu must learn in order to advance further. Each subsequent rank requires a specific lesson to be acquired, usually from experience. The spinagons therefore must learn that it is best to act (and succeed) as quickly as possible (in order to avoid any unnecessary hassle).
Fig. 4: Spinagon (Spined Devil)
Now, a good spinagon (well, not good good, of course, but resourceful and hard-working) may have a chance to advance to a higher rank: be it abishai, barbazu, osyluth, hamatula, or amnizu. Or, if he's a bad spinagon (surely you get it by now), he might end up a lemure again. But, ah, let's not get too carried away. This is enough of a lesson for today, cutter. More on the devils another time, eh?