(note: this was just something I came up with during a bike ride. It may or may not be canon. Characters belong to @dmitrysfutadotcom)
There exists two distinct strains of horsegirl genes: dominant and recessive. While they share many similar traits, such as a high sex drive, there are some key differences.
A woman with dominant horsegirl genes exhibits certain physiological traits. The first and most obvious is that they possess large equine genitals. These can vary in size and shape, but they are always a penis and testicles. Second is a horselike tail that grows from their coccyx and matches their natural hair colour. As with horses, they can exhibit some control over their tails’ movement. Unlike horses, their tails don’t typically communicate their state of mind, except in the case of particularly strong emotions. Some horsegirls take to styling their tails like they do their hair. The ears of a horsegirl naturally come to a point, though this has no appreciable effect on their hearing ability. Horsegirls are also predisposed to greater than average physical strength and endurance as well as higher bone density.

(fig.1, Sam, a woman with dominant horsegirl genes)
Conversely, a woman with recessive horsegirl genes can appear quite common up until their adulthood. They lack the tail and pointed ears of their dominant-gene sisters, and have physical attributes that can vary as much as an average human. Their only similarity is that they also posess a penis and testicles, though theirs are within the normal human range. This changes, however, in adulthood, when their sexual hormones peak. Their bodies undergo a “second puberty”, that is, a further development to their horsegirl physiology. Over a few years, their genitals grow and transform to an equine size and shape to match the average horsegirl. This period also coincides with a time of intense sexual desire as their equine hormones assert themselves.

(fig.2, Katherine, a woman with recessive horsegirl genes, post-second puberty)

(fig.3, Scarlett, whose recessive horsegirl genes have yet to assert themselves)
Interestingly enough, non-horsegirl offspring sired by a horsegirl will possess strong physiological affinity and sexual compatibility with horsegirls. This can be expressed a number of different ways. The most common is simply exhibiting a higher than average fertility in the presence of horsegirls, though the way it can be expressed varies from person to person.

(fig.4, Courtney, a non-horsegirl male whose body nonetheless reacts to the sexual presence of horsegirls)
In terms of offspring, children sired by a woman with dominant horsegirl traits will be approximately 50% dominant, 15% recessive and 35% non-horsegirl. A woman with recessive horsegirl traits will sire approximately 80% recessive and 20% non-horsegirl.




