Biology Overview: Succubus
As with all “supernatural” monster girls the Succubi obviously have no real magic powers, nor demonic energies, nor infernal heritage. Rather the Succubi were designed to be the social butterflies and actors of the monster girl races.
Succubi feature a largely humanoid body structure with the addition of a prehensile tail and bony spurs that can develop into horns in several areas from the forehead to the temples. There're additional very tiny spurs on the shelf of the jawbone that are not visible. These horns and bone spurs serve as anchor points for their large number of specialized facial muscles.
Succubus skin contains a vast number of tiny muscles, especially in the head and face, allowing them to reshape their skin's texture and change their apparent facial bone structure. These muscles are actually present in all animals, used to jerk the skin across the muscles (try lighting touching a cat and see how its skin twitches) but are atrophied in baseline humans. Succubi re-strengthen the muscles and use them for minor shapeshifting tricks. They also have chromatophores and photophores in their skin granting them the ability to change color and even glow. Succubi cannot change their hair color via chomatophores but all succubi have naturally pale hair that accepts dyes readily and looks like naturally colored hair even when dyed electric blue. Somehow.
Together these abilities let a succubus appear to be any race, gender, or phenotype at will as well as letting them subtly change their apparent body shape, bust and rear size, and similar measurements. It's not exactly shapeshifting but it's a far better disguise system than any baseline can manage, and can be done in seconds. Succubi can also use their chromatophores to give themselves makeup and “tattoos,” most succubi will have a visible ink whenever it's socially acceptable and the more complex and multicolored her “ink” the better, as it's a boast of just how good her control over her own body is and her skills at color shifting. Highly skilled (and boastful) succubi may animate their tattoos and make them glow neon colors.
Succubi have slightly enlarged pointed ears to improve hearing, needed for their mimicry skills (Also easily hidden under hair or a hat), though their internal ear structure is nowhere near as radical as an Elf's. Combined with their tail, they have balance abilities as great as an elf's, but as the tail is involved, they can't match an elf in a BattleMech where the inner ear is the only part that counts. Succubi also can't balance as well if their tail is constrained by clothing in an attempt to pass as human. In some cases succubi have had their tails surgically to more easily pass as human, though like any monstergirl body part it will regrow with time.
A succubus's horns have more similarity to deer's antlers than the horns of species like cattle or goats. When her hormones shift in a specific way, she grows a sheath of blood-rich velvet that supports the bony growths on her head, then rubs away once the horns are fully grown. Apparently, this is incredibly itchy. Hormonal treatments can allow a succubus to shed or regrow her horns as needed (though it takes some weeks), and braces along with growth treatments directly on the velvet can let her direct the horn shape into anything from a single discreet prong to curled horns imitating rams to multi-spiked growths several feet wide. Horns can theoretically be used as weapons but few succubi would bother if they can put their hands on a gun.
Lastly succubi have a complex vocal cord arrangement combining both a larynx and a syrinx, and similarly to a lyrebird they have a tremendous range of vocalization. Not only are succubi excellent singers but they can effortlessly adopt most accents, duplicate noises like engines running or a zither playing, and even imitate voices and sounds well enough to fool voiceprint computers. Combined with their color change and facial restructuring muscles a succubus can make an incredible infiltrator and don disguises at a whim.
Socially, succubi refer to their horns as their “rack.” This is annoying to other monster girls, who use the term to refer to their busts. This is especially annoying because succubi still use “rack” to refer to their busts. I suspect Pants started that meme just to annoy everybody else. A succubus who grows her horns out long is referred to as a “coat rack” while a succubus who sheds her horns to look more human is a “hat rack.”
