My first stab at a fan-made:
Lesser Succubi are created when a Succubus has sex with a female human but fails -- or is unable -- to infuse enough Dark Mana to change her all the way.
In turn, Minor Succubi are the result of a Lesser Succubus attempting the same feat. Since most of the Lesser Succubi's mana is bound up in her own transformation, there is very little left over to infuse into a human woman, nowhere near enough to create another Lesser Succubus.
Instead, the result is a Minor Succubus. Minor Succubi have all the lust and lasciviousness of their Lesser and full Succubus counterparts, but are not even as transformed as a Lesser Succubus is. A Minor Succubus lacks wings and tail, and her horns are so small her hair typically hides them completely. Aside from her horns, a minor boost to physical attractiveness, and a possible age regression to young adult, she looks exactly as she did as a human.
The mana of a Minor Succubus is much more limited than that of a Lesser Succubus, the majority being dedicated to furthering the transformation and as a result, Minor Succubi have no charm magic available to them. They instinctively know the basics but have so little available mana to work with, their attempts just fizzle.
They are mamonos however, and they have the same compulsion to mate with men that any Succubus does. Their urges are even more intense than those of any other Succubi however, as they instinctively know they will get stronger from sex and desperately need all the power they can get.
Where many mamonos are physically stronger or faster than humans, Minor Succubi are limited to whatever strength and speed they had as humans. Their stamina is initially improved to that of a top notch athlete, though they lack the superhuman stamina of the typical mamono. Over time, all three will improve as their power increases.
Despite the superhumanly boosted libido, Minor Succubi are the closest to human out of all the Succubi subspecies in their morals and desires. Their new instincts and supercharged need for sex will drive them to lengths they would never have considered as a human, which will at first alarm and possibly even horrify them. But eventually they will adapt to their new species just as Lesser Succubi do.
Minor Succubi cannot become pregnant, their bodies are too focused on continuing the transformation to waste any energy on it. Their bodies will adjust to become more attractive since the more men they seduce, the more energy they will have to continue the change but once the initial burst of change is complete, they cannot alter their appearance as other Succubi can.
Minor Succubi will eventually evolve into full Succubi, skipping the Lesser stage, though it takes them a lot longer to get there than a Lesser Succubus does, since they start off with much less power. They typically do not manifest the wings or tail of a Succubus until they have collected enough spirit energy to complete the transformation. Their bodies get so monomaniacally focused on collecting energy to complete the change that the actual completion nearly always comes as a surprise -- both to the Minor Succubus and the man she's with at the time.
if she lives in a Demon Realm or eats Demon Realm food -- or both -- she will manifest larger horns, wings and tail long before the energy collection is finished, as the more abundant environmental Demonic Mana soothes her instinct to conserve mana. While she will never go fluffy like a Lesser Succubus, her wings and tail initially do manifest as translucent, like those of a Lesser Succubus.