I am working on the development of a humanoid species. I figured they'd have split along the evolutionary chain around the same time as humans. They'd live in a warm climate, resulting in little hair any where, but their head, since eyebrows and lashes protect against debris and head hair protects against the sun. I also gave them horns, a tail, claws, long canines, black scleras, and grey skin. They each have both sets of genitals, but the 'boy' part is sheathed, and their bodies adapt when they become pregnant. I think I have a decent backing to each evolutionary trait, but I want to know if it seems too unrealistic.
When looking at whether a trait makes sense, the important thing is to consider if it would help the animal go about its life. Many of these traits exist, or at least, there are genes that could cause them to exist, in humans. However, I'm not sure they all make sense in the same organism. First, let's go through all of the traits that would be perfectly fine in a humanoid.
Grey Skin There are grey skinned animals out there, so this is a possibility. I'm not aware of any particular downsides to having grey skin, either.
Black Sclera Most of our close relatives, evolutionarily, actually have dark or black sclera. Chimpanzees are a prime example. It's hypothesized that we evolved white ones to aid in communication, but it's perfectly reasonable that this mutation never occurred in your creatures.
Then there's a few evolutions you've listed that make sense, but require some shifts to the way your humanoids will behave:
Claws & Teeth
These are reasonable things to evolve, assuming that your humanoids hunt with them. Heavier canine teeth are good for eating meat, and claws can be powerful weapons for catching prey. For large animals, nails might be worse for climbing, since they get in the way of gripping around small branches, but if bipedalism evolved in a small climbing creature instead of a large one, they might evolve.
This could exist in a humanoid, particularly one that actively uses arms for fast hunting. I'd expect a tailed humanoid to have evolved to sprint down its prey and dispatch it on the run with swipes from its claws, necessitating the evolution of a tail for balance instead of using the arms, as humans do.
The other traits are less likely to evolve. Yes, humanoids could potentially have the genes to create this things, but ultimately, it seems like evolutionary pressure would favor individuals in the population in which these traits were reduced or absent.
A predatory creature with claws and large teeth is unlikely to evolve horns. Unlike herbivores, carnivores have evolved to kill things, and don't tend to need to evolve extra weapons like horns. Furthermore, effectively using horns would require that the creature put energy into growing them, and into growing and sustaining the supporting structure needed to make them useful. Horned creatures need strong necks to support use of their horns, and the neck musculature wouldn't be critical to anything else for a creature evolved to run down and claw up its prey.
This is unlikely if your humans raise young in pairs or in groups. OR rather, it's unlikely that the same individuals would be able to function both as a male and as a female at the same time. The more organs and structures an animal has, the more energy it requires to live. Maintaining both sets of reproductive organs, and having both be functional at the same time, would require that the creature consume more food than if they focused solely on being male or female. Truly hermaphroditic animals, like snails, some worms, and brine shrimp, generally don't put a lot of effort into raising their offspring, and as such can increase their reproductive fitness if most of the population can be simultaneously impregnated.
Sequential hermaphroditism, on the other hand, would be much more feasible for a group living child-rearing creature. Sequential hermaphrodites are born either male or female, and then switch later in life, generally with the most dominant creature in the group becoming the other gender. If your humanoids were born exhibiting female traits, this would allow the fittest of the creatures to share its genes the most, since only the most powerful of the females would be able to become male, increasing the number of offspring it could parent at a given time.
It would also make sense for your creatures to become truly hermaphroditic when solitary for extended periods of time, since this would allow them to self-fertilize and reproduce even in the absence of anybody else.
The main issue is the hermaphrodites, the other properties are extensions of existing mammalian traits so should be fine.
Hermaphrodites though are much harder to explain. In particular a lack of females would not explain hermaphrodites at all, if a species lacks females then it dies out - but it can't prompt boys to turn into girls to balance the sides.
I think a more plausible explanation would be to have occasional fertile hermaphrodites to have occurred spontaneously. For whatever reason they were seen as sexually desirable or even worshiped by the other members of the species. As a result those hermaphrodites got to breed, a lot.
When the hermaphrodites started not only having children themselves but impregnating females then those properties would spread rapidly through the population. You might even have a temporary phase where 3-way marriages are common (one male, one herm, one female) in the society.
Either way over time things would shift until hermaphrodites were the most common members of the species, while male and female would both be rare throwbacks.
It's out there, but let's try to see what we can do.
Horns - Since they share a common ancestor with humans, and there are humans born with horns, it is possible that those of your species who were born this way, were highly revered and bred very often, at the expense of others. Soon with each other as more of the genetic mutation started showing up, it resulted in evolving to popularity, eliminating any 'freak' born without horns over the millennia.
Tail - Same as above.
Long Canines - Wow, same as above.
Grey Skin - Same as above
Genetalia - Same as above, but sometime later, the chromosomal genotype became the norm, and it was very rare for an XX/XY to be born. The more 'successful' interexed (those with very distinguishable double genatalia), were selected naturally. If you want a person to be able to breed with themselves, I can't help you there - I think your species would suffer tremendously from that.
Black Sclerae - Scientists belive we have bright eyes with whites, because it helped with communication, unlike our evolutionary cousins. In the case of your folks, they maintained the dark eyes of our common ancestor, because (taking a stab here), certain survival behaviors depended on not knowing where someone is looking. In your prehistoric times, tribal warfare was a staring contest: as soon as the opponent looked away, they got gored by the horns. So, over time, those with darker scleras were more successful.
One thing however, I'm not sure that we would both evolve so distinctly as we would compete, but then again, we share a common ancestor with apes.
This is not too unlikely, except for the sheathed penis, the rest can be seen in some form in regular humans to one degree or another. I take it from your apparent discomfort with the word penis, that the required 'sheathing' was for the same reason. Don't be afraid, if you let them rock out with their cocks out then it's not a terribly unlikely creature. You can check out some diagrams of hermaphrodites for what to expect, like any naked man, it's not going to be pretty.
This is not going to likely come from the human genome. There are cases of people with horns. But they're most likely caused by exposure to radiation (or perhaps a virus). In either case horns can be added to children in a ritualistic way, similar to the artificial cranial deformation performed by some groups of early humans. The virus is easier, since the sites could be infected until the virus takes hold. For the radiation case, it's less likely to work so frequently. Unless they are particularly disposed to that reaction and a reliable source of radiation is used, the normal source is the sun, but perhaps they have some ceremonial lumps of radioactive rocks.
There are cases where humans were born with tails. So the trait exists in our DNA in a way capable of being expressed. While this is quite rare, it could have been a desirable trait in this species.
What are claws besides thick fingernails? Not a difficult adaptation to believe.
This, again, is not a huge deviation from normal humans, so it not terribly unlikely or hard to believe.
This is not going to be very satisfying. Fully black scleras don't occur. The best way around this is to have very dark and large irises, like a canine. These would appear like black eyes, but the scleras would still be white.
Humans come in all colors of the human colored rainbow. Grey is not very specific, but I don't see any reason the species could not appear grey compared to a normal human.