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 monsterization, 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 too do incubi, with increases to lustiness, physical attributes, and lifespan.

Incubi are said to be "an ideal male to a female monster", and 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 dispoal, like dark matters or khepri, are noted as being able to effect the change quickly, but the majority of monsters must just rely on repeated, repeated, repeated sex. Certain items that can expedite the transformation, like succubus nostrum, may be sought by eager wives.

A Being That's Convenient For Monsters

 * Refer to: Fallen Maidens; The Incubus

Physical changes
Besides genital gargantuism, incubi do not recieve drastic physical changes comparable to, say, an arachne's abdomen or a succubus' wings and tail. They do, however, see away any features that impeded their sex lives previously, like corpulescence or the general frailties of old age. And as the time spent with their monster partner increases, incubi find their appearence, and even their apparent age, adusting 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 profound changes under the skin for an incubus too. Stamina increases to monster-like levels as the eldritch power of demonic energy suffuses the muscles, 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 sex in place of food.

Mental changes
On first being attacked by a monstergirl, humans are likely to be consumed by an instinctive fear and revulsion at their demonic grotesquery. 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 dick, and interest in other things (like work) decrease in tandem:

Incubi never get bored of sex or lose the mood after doin' 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 get satisfied unless he has sex with all of the monsters in the harem".