Sylph
Sylphs (Homo sapiens alfus) are an offshoot subspecies of humans originating from the gas giant moon Frey. They are descendants of the crew and passengers of the generational ship UES Ymir. Over thousands of years, they genetically modified themselves to better adapt to Frey's unique environment.
Biology
Sylphs have a similar body plan to humans but with a few notable differences. Sylphs tend to have pale skin, pointed ears and a lean build adapted for Frey's low gravity compared to Yeddo. Most Sylph tend to average around six feet in height. On their feet, they only have two toes, but otherwise have five digits on their hands.
One of the most prominent features of a Sylph is their two large, vertically-oriented horns made of keratin. These horns grow continuously throughout their lives, and are shed around once per year. The shapes of the horns are unique to every individual, but Sylphs that have similar patterns of horns tend to share similar ancestry. For example, Sylphs born on Frey have differently shaped horns than those born on Yeddo.
When exploring other worlds, Sylphs that are native to Frey must intake oxygen from a breath mask once every four hours. This is largely due to Frey being much richer in oxygen the standard atmosphere breathable by common humans.
History
Early history
During humanity's early spacefaring era, many generational ships were sent to explore the Milky Way galaxy - most of which had their courses charted for various planets and moons identified to have an oxygen-atmosphere and be located within its star's habitable zone. The gas giant moon Frey was one such world that was identified, and the UES Ymir was sent to explore it. Early long range surveys indicated it had a diverse, jungle ecosystem which made it one of the most popular candidates for exploration.
When the UES Ymir arrived at the moon, it was quickly discovered that despite it having a nitrogen-oxygen atmosphere, the specific concentration of gases was considered lethal and would cause oxygen intoxication within minutes for anyone without a breath mask. Unfortunately, the UES Ymir was not designed for relocating to another potentially habitable world so the colonists were forced to make due and settle the world. They still used the ship as a main base of operations while building sealed off environmental camps for studying the local flora and fauna.
After twenty years and the start of the next generation, the colonists were faced with a difficult predicament. The infant mortality rate was unusually high due to environmental complications. To solve this, the colonists found themselves having no choice but to start using genetic engineering to ensure the survival of the newborns. The first major modification was adjustments to their respiratory system so that they could breathe within Frey's atmosphere. The second was modifications to their bone structures to resist bone mass loss due to the world's lower gravity. And the third was adjustments to their digestive systems to better process food obtained from Frey's environment.
As the generations went on, the genetically modified humans soon began to outnumber the unmodified ones as they became more suited to Frey's environment. They were able to not only adapt but thrive. The colonists evolved to adapt to Frey. They were able to expand well beyond the landing site of the UES Ymir which was eventually abandoned and left to be claimed by nature.
Evolution of the horns
Around one hundred years after the colonists began settling Frey, some of them began to exhibit a genetic anomaly where they began to sprout horns. This proved to be harmless towards their biology and likely was a result of their genetic experiments. However, these horns quickly became a topic of discussion. The colonists began to wonder whether or not they were still human. Around this time, they had begun to call themselves the Sylph as they had begun to develop a society vastly different than the original crew of the UES Ymir. Some viewed the horns as a mark that the Sylph had transcended humanity. Others saw it as a genetic quirk with no bearing on their lives. While others saw it as an aspect of evil.
Nevertheless, the horns prevailed evolutionarily for whatever reason. Because the horns took on different shapes, many Sylph began to align themselves into clans based on their ancestry and similarities to one another. Rivalries formed and competition for resources intensified to the point of violence. While this led to division among the Sylphs, it also encouraged them to spread out further across the world so as not to overlap on one another's lands.
Contact with the Valerian Empire
Several thousand years after the UES Ymir landed on Frey, an envoy from the Valerian Empire was sent to make contact with the Sylphs. The Valerians had explained to them that they were attempting to track down all the original generational ships sent throughout the galaxy in hopes of reuniting humanity. The Sylphs, however, initially rejected the Valerians' attempts at assimilation. Society on Frey had become fairly isolationist by this point. Many had come to believe they no longer had anything to do with humans and were now their own species.
However, some younger Sylphs expressed interest in the Valerian Empire for the sake of wanting to see the rest of the galaxy. Several hundred Sylphs volunteered to go with the Valerians as diplomats and ambassadors. However, many of the Sylphs that remained behind grew suspicious. They believed the Valerians were a potential future threat that could put their culture at risk.
Post-Blackout
Even after the Blackout, many Sylphs still remain living in an isolated society on Frey. Although vrNet servers to their homeworld exist, they remain incredibly wary of outsiders, often exhibiting xenophobia around humans. However, this is not the case for all Sylphs. Some populations which had intermingled with the Valerian Empire in the past produced descendants that could breathe in a standard nitrogen-oxygen atmosphere breathable by humans. These hybrid Sylphs have fully integrated into Federation society.