Centaurs come from a planet fairly similar to Earth, and their biology is convergent with Earth’s in many ways. Their evolutionary ancestors were savanna pack predators who used ambush and speed to hunt prey. Modern centaurs emerged when they started to use tools like spears to help with hunting. Centaurs are obligate carnivores but they supplement their diet with grain, starchy tubers, and small amounts of roughage and vegetation. Much like humans, they have a bisex reproductive system where the individuals with the larger gametes give birth to live young.
NOMADS AND SETTLERS:
Centaurs have many ethnicities, but there are two major groups divided by lifestyle. Although they have many notable differences in physiology, they are not considered separate species as geneflow between the two groups is fairly high.
Nomads tend to be smaller, stockier, with shorter spines and higher endurance. They have a more “canine” lifestyle, walking long distances driving and livestock and wild stock prey herds as they go. In bare-handed hunting they tend to favor organized sprinting pursuit, similar to wolf prey capture.
Settler centaurs tend to be larger, with longer, more flexible spines and higher sprint speeds. They have a more “feline” lifestyle, with clans occupying fixed territories of livestock and wild prey. In bare-handed hunting they tend to favor stalking followed by short sprints, like cheetah or lion prey capture.
Centaurs are pseudo-eusocial, with a social structure hierarchy somewhat similar to meerkats. At its most basic level a clan consists of one matriarch, a female who is responsible for bearing the clan's young; the entourage, who are the matriarch's partners and usually mostly male; and the clan's workers, who are not involved in reproduction. Females are larger and stronger than males on average, and worker females are usually involved more in labor tasks and males more in childrearing. Centaur males are more likely to have a visual "cute" reaction to infants and animals than females. There are no major physical differences in centaurs with different clan roles.
Female centaurs have a very slow estrus cycle by default, and are only actually capable of becoming pregnant about once a year. Getting pregnant routinely speeds that cycle up to several times a year. More detail on the reproduction page. Other centaurs can usually determine estrus (and thus matriarch status) by scent. Although partnerships between workers are fairly common, in more conservative clans sex may only be permitted between homosexual couples, or outright celibacy is enforced.
Centaurs who regularly experience pregnancy experience innate aggression towards infants they didn't birth. Matriarchs will usually respond to other members of the clan becoming pregnant by either ousting them to form their own clan, or infanticide. Nomads have especially strong infant-aggression and a propensity to smaller clans, while centaurs in urban centers tend to have lower infant-aggression. Some urban centers even have multi-matriarch clans with a shared worker and entourage pool, though the different matriarch's litters are usually raised in separate buildings to prevent irritation.
The main airway is unidirectional, except for the occasional sneeze. Air enters through the incurrent nostrils on the face, passes into the first air chamber, then to the main lungs that surround the heart and braincase, and then exits out the excurrent nostrils/vents on the rear of the lower ribcage. The majority of oxygen exchange and moisture reclamation happens in the lower lungs, which have a gill-like, high surface area comb structure that the air passes through. Centaurs cool themselves mostly by panting, where they breathe rapidly and open the lung structure up wider in the chest to allow more air to bypass the comb and leave with water and waste heat. They can produce subsonic hums, rattles, and groans out the excurrent nostril by clenching the valve.
The centaur vocal apparatus evolved from an olfactory structure that got co-opted for language. There are still scent receptors in it, but the thing has become much more elaborate than in their ancestors. To speak, they breathe into two large sinuses on the back of their head, close those off from the main airway, and compress them to push air out through the trunk. Of the aliens in Runaway to the Stars, centaurs are probably the best at vocalizing human languages. The phonic structure is pretty similar to ours, consisting of vocal cords set in the skull at the base of the trunk, resonant loops back to the incurrent nostrils, and muscular ridges inside the trunk towards the lips. Their large toothed mouth is totally unconnected to the respiratory system, it only leads into the esophagus.
Centaurs have two seasonal bony structures that grow from their lower jaw.
Centaurs are generally crepuscular, sleeping twice a day on average. The night and day cycles on their homeplanet can be incredibly variable.
Centaurs have six teeth that continuously grow from the root and shed scales/plates from use. This serves a self sharpening and cleaning mechanism. The front two teeth form an incisor beak for hooking prey or cutting food, and the rear four function like molars or carnassial teeth, useful for grinding food and crushing bone. Their teeth can be pigmented anywhere from light pink to dark purple, which varies based on local genetics.
Centaurs have their primary brain in their mid torso and a much smaller secondary brain in their head. The primary brain handles higher cognitive function, while the secondary brain deals with immediate sensory processing and fight-or-flight reactions. Also, unlike us, their nervous cord runs underneath the mass of the spine. This is much safer and more reasonable than its placement on humans; our nervous cord runs along the outer side of the spine. Their first pair of limbs are attached to a single large girdle bone, like if you had a pelvis in your neck.
The centaur setup with the cognition-focused midbrain sitting in the midsection and a fight-or-flight/sensory ganglion at the very front end also means they are more prone to various nervous degeneration issues in their old age. Diseases like MS are bad enough for humans with our main senses right next our brain, in centaurs the resulting communication delays down the spine can cause some pretty debilitating behavioral and sensory conditions.
Centaur integument is comparable to feathers, but they are not that similar on a structural or developmental level. The surface feeling of their velvet would be similar to horse fur, and their long coverts more like big cat fur– but if you dug your fingers in you would feel the densely packed, thin quill bases and bird-like down. They shed continuously and have oil glands at the bases of their quills sort of like Earth mammals do.