Incubus

An incubus is the male counterpart to a succubus, and represents the fate of (most) males exposed to high levels of demonic energy. Unlike female monsterisation, where the physical and mental change from human to monster is rapid (in some cases taking only moments ), for incubi the process is much more gradual and open-ended. Just as monstergirls become more more powerful the more sex they have, so do incubi, with increases to lustiness, physical attributes and lifespan.

Incubi are said to be "an ideal male to a female monster", therefore many monsters are eager for their mates to transform faster so they can enjoy the changes. Monsters with a lot of demonic energy at their disposal, like dark matters or khepri, are noted as being able to effect the change quickly, but the majority of monsters must just rely on lots and lots and lots of sex (oh noes). Certain items which can expedite the transformation, like succubus nostrum, may be sought by eager wives.

Although the vast majority of information on incubi pertains to human men, elves can definitely become incubi and there seems no reason that any of the few male dwarves left can't do so as well.

A Being That's Convenient For Monsters

 * Refer to: Fallen Maidens; The Incubus

Physical changes
Besides genital gargantuism, incubi do not usually receive drastic physical changes comparable to, say, an arachne's abdomen or a succubus' wings and tail, although their eyes change color due to demonic energy. They do, however, see away features that impeded their sex lives previously, like corpulence or the general frailties of old age. And as the time spent with their monster partner increases, Incubi find their appearance and even their apparent age adjusting to match their wife's taste: "It could be as a boy, as a young man, or as a mature man". Since this happens simultaneously with the man's tastes reshaping her, eventually both monster and incubus will gain their spouse's ideal fantasy body.

There are changes under the skin for an incubus too. Stamina increases to monster-like levels as the eldritch power of demonic energy suffuses their body, and incubi gain some measure of the environmental resilience of their monster partner (e.g. heat resistance for a lava golem's mate, breathing underwater for a Sea Bishop's mate). Like a succubus, they also gain the ability to actually sustain themselves on their partners spirit energy in place of food.

Mental changes
On first being attacked by a Monster-girl, humans are likely to be consumed by an instinctive fear and revulsion at their demonic appearance. Such atavistic terror and Order moral objections are thrown aside with the transition to an incubus: Their thoughts and values change, all becoming centered around having sex with monsters. Lust skyrockets to keep pace with the monsters' insatiable hunger for cock, and their interests in other things (like work and hobbies) decrease, as sexual activity becomes a more important part of their lives:

Incubi never get bored of sex or lose the mood after doing the deed; they just keep going. Whereas a human first being attacked by a monster is likely to pass out from pleasure before his rapist is close to being finished with him, incubi flip the dynamic on its head and "end up being unable to be satisfied, unless he has sex with all of the monsters in his harem".

Magical changes
When women undergo monsterisation, their spirit energy production ability (which is weaker than men's to start with) gets extinguished entirely and they become a creature with only Demonic energy inside them. The same does not occur with men and incubi, instead their is a large improvement in mana production in response to increasing amounts of Demonic energy within their bodies.

The Demonic energy that enters a man, "marks" an incubus with his partner's uniquely scented Mana. Whether other monsters chose to respect this implicit marking of territory is down to their personal tastes; unicorns and cancers could be discouraged, while beelzebub and bicorns have the opposite reaction. Although human women can't detect it, the demonic energy can have a drastic effect: if an incubus has sex with a human woman, his semen will monsterise her.

Daily lives
Although the sex drive of a man who has become an incubus is greatly enhanced, causing sex with their monster partner or partners to be a greater part of their life, incubised men are still free to pursue previous hobbies and work from before incubisation. This often results in monster nations exporting produce to other nations, sometimes producing goods that can only be created by a monster and her husband, or are only available in Demon realm lands.

A large cultural shift has begun amongst the monsters more recently, with monster wives taking a keener interest in more human activities, whether it be culinary or a craft. Many of these changes have occurred as monster women have sought to improve the lives of their husbands, helping to keep them happy. Many single monster women have also taken up crafts to further impress potential mates, instead of directly assaulting them.