Warning: I am going a bit into teratology here. If you google up the stuff I mention, you may find graphical things that can cause the most sensitive among us to lose a few nights worthy of sleep. I have abstained from providing links to those (otherwise what I'm linking to is sanity-safe). Look them up at your own caution.
Also: thanks to Astrid_Redfern for some extra info I have added to the post, regarding Shope papilloma virus and wing growth, eye and skin coloration, and behaviour effects of a certain parasite.
Is this plausible in some way, or totally impossible with rules of our world?
It is plausible. It is improbable that such a creature would would evolve in our world as it is now, but evolution might have gone differently.
Some people are born with tails. That's because some tail building genes are present in the human DNA, although deactivated. A mutation causes the genes to reactivate. Usually these people have their tails cut off shortly after birth. If we as adults were able to have more totipotent cells - that is, cells able to produce all kinds of differentiated cells - a tail could be grown after birth. The genetic activation might be provoked by a microorganism such as a virus, or a larger one like a worm could do it by injecting some hormones into the host.
An example of a parasite (in this case, a virus) causing something to grow in humans is epidermodysplasia verruciformis, a condition whose sufferers are called tree men. The growths are so huge that they frequently become larger than the person's fingers, rendering the hands useless.
Now I am quite sure we don't have the genes for horns (the horn in some cyclopic babies doesn't count as that is just flesh, also don't google it), though we do have the genes for increased amounts of keratin and bone, which is what horns are usually made of. For keratin, look at harlequin babies (warning, nightmare fuel). For bones, see osteochondroma.
Notice that leporids (bunnies and rabbits) are susceptible to infection from the Shope papilloma virus, which can lead to keratinous carcinomas (cancers) resembling horns. These usually grow on the head of the animal, in places where there is already hair growth. This could be your succubi/incubi horns right there, and might be the explanation of why the condition is unhealthy.
For this it might have to be a world where humans have some common ancestry with winged creatures. It's your world, and it's touching on the supernatural - you could justify it as people being descendants of angels, or true demons.
Alternatively (and perhaps more realistically), their wings might be like the leathery wings of a bat. A papilloma virus that makes the body produce just the right kind of skin might do the trick, connecting the wrist to the ankles. I believe such wings could even be functional - not for true flight, but for gliding. I have discussed this in another answer about human gliding, and also this one about human wings.
A bone condition known as osteogenesis imperfecta usually causes the cornea to have different pigmentation. This condition was made famous due to a trilogy of movies starring Bruce Willis (starting with one called Unbreakable). There is a picture of the effects on the eyes of a person in the wiki. It is one of the things in my list that is not disturbing, it just gives the cornea a different hue (though sometimes it also causes a slight eye protrusion). For the iris, some diseases such as Fuchs heterochromic iridocyclitis are known to cause discoloration. More causes for iris color changes can be found in the latter link.
Pigment dispersal syndrome can also cause eye color changes, with less short term adverse effects if you wish to tone those down. Long term it causes glaucoma and then blindness, though.
Something extra about eyes
It is said that people are seen as more beautiful when their pupils dilate. In the past people used belladonna (a poisonous plant) to achieve this effect, because humanity. In fact the name of the plant means "beautiful woman" in italian. I find it debatable whether stoned eyes are beautiful for today's standards, but if it beats their kink, it beats their kink.
Your succubi/incubi might have permanently dilated pupils. It will be seductive for some people, scary for others, and generally give them slightly better vision in low light or underwater.
Trigger vitiligo for a paler skin. Or trigger porphyria, the vampire disease to make the afflicted so much sensitive to sunlight that they will get burns from levels of exposition that wouldn't tan healthy people. Avoiing the sun will make them paler. If you want people to be "pinker", rush blood to the skin. This might also cause their skin to feel warm to the touch.
Notice that all the conditions above are harmful and mostly unpleasant to the eye, but with the right evolutionary pressures you could have a more perfectly "demon" lookalike. If a succubus/incubus appearance causes the person to have more offspring, for example, evolution will push appearance that way. Why not everyone is a succubus, then, might be because either such people are killed due to superstition, or because the agent that causes such changes also causes a lot of harm in some other way. If all the agent did was beneficial to the host, it would be mutualism or commensalism, not parasitism.
Finally, succubi and incubi are known for either requiring sex to live, or to obtain strength. It might just be the stuff of legends, with the hormonal changes caused by the parasite resulting in a second puberty (except that now you are above legal age, so hooking up with people is easier).
Toxoplasma is a parasite of cats that is known to thrive in humans. There are many papers that say it affects human behaviour, and some that say it doesn't. Whether it is true or not in our world, it can be true in yours. Here is what a paper published Schizophrenia Bulletin of the Oxford University has to say about it:
Consistent and significant differences in Cattell's personality factors were found between Toxoplasma-infected and -uninfected subjects in 9 of 11 studies, and these differences were not the same for men and women. After using the Bonferroni correction for multiple tests, the personality of infected men showed lower superego strength (rule consciousness) and higher vigilance (factors G and L on Cattell's 16PF). Thus, the men were more likely to disregard rules and were more expedient, suspicious, jealous, and dogmatic. The personality of infected women, by contrast, showed higher warmth and higher superego strength (factors A and G on Cattell's 16PF), suggesting that they were more warm hearted, outgoing, conscientious, persistent, and moralistic. Both men and women had significantly higher apprehension (factor O) compared with the uninfected controls.
It is also known to drive testosterone levels up in male rodents. If a microbe is able to do that, it might also be able to drive a person's sex drive up given the necessary evolutionary pressures.