I've pretty much always been a fan of Fey, but only within the past six years or so have I actually started to truly love them. And that can be traced 100% to the sorts of authors and book series I read. Jim Butcher's Dresden Files. Seanan McGuire's October Daye series. M. Todd Gallowglas's The Tombs. A Midsummer Night's Dream. There are other major non-literary influences as well - the old Gargoyles TV show with its Puck and the other Children of Oberon are also a major contributor, for example - but these books that I keep coming back to are the core of what has defined the Fey for me. When I designed the Courts of FaeReie for Finiens, it was these stories I had in mind. Combined with my love of mythology and things like Grimm's Tales, there was no way Finiens was not going to have a heavy Fey influence; doubly so thanks to the first campaign ever being run in our world being Kingmaker.
As such, it seemed only natural to have Fey races available for play. Sadly, fey creatures scaled for PC-level stats are few and far between; even the weakest of fey creatures has a statblock with abilities or resources that would be immensely unfair (and a headache to GMs) to put in player hands, more often than not. That said, it had been done in the past; in 3.5, one of my favorite PC races were the Killoren from Races of the Wild, the green-skinned "humans of the fey" with their shifting aspects. I actually played a Killoren character in a short-lived campaign, an Archivist named Shezzarin. She and her party are actually somewhere in Finiens's history thanks to their plane-hopping shenanigans, as are a time-traveling party from another campaign my group ran, but those are stories for another time!
I'd wanted to have a satyr-like PC race for some time, I'm not exactly sure why. They were called "satyrs" at first, too, with females being referred to as "nymph satyrs" despite retaining the half-goat appearance of the males. However, that idea was scrapped when it became obvious that using the "satyr" term would cause confusion with the actual Satyr creature already in the statblocks, as well as lock in the race to all the scandalous sexual behavior associated with that particular breed of fey, which was not something I wanted. So I did some searching around for another creature that had a similar physical design but a different lore. Fauns existed, but they too were already statted out and part of the game as-is. Then I found the glaistig, a fey/ghost creature from Scottish myth.
The appearance was perfect - they had the goat legs, but their upper humanoid half was elf-like and, to my surprise and amusement, lacked the horns almost always present on Satyrs and Fauns; this soon became the primary physical differentiating trait between the Glaistigs and the other half-caprine fey. At least according to Wikipedia and the other quick website sources I found in my early searches, Glaistigs were either a protecting spirit, sheltering herds and handlers from wolves and raiders, or an angry avenger of wrongs done. I decided to blend the two and turned them into FaeReie's first guard, a species created for the purpose of defense of the weaker and engaging the invading enemy first, and designed their statistics to match, giving them strong builds, hardy immune systems, resilient skin, and warding their minds and presence against magical intrusion.
At the time I created them we really hadn't decided what to do with Gnomes yet - that was only decided about a year and a half ago, partway through Kingmaker - but we had decided that the primary enemies of the fey were a shadow-fey species (which later became the Lost Court) and the Dark Song, which I'll elaborate on eventually. We'd also already decided that the abandoned realm of Unknown Kadath was a place (one of two major such locales) where the Dark Song could enter the world from whatever netherworld they originated, which later became FaeReie though that is far from their point of origin, and attack the mortal realm. Thus we positioned the Glaistigs who had left FaeReie at the feet of the Mountains of Madness in the Ice Claw, standing first sentinel against the Song incursion.
The Glaistigs are only the first of the fey races that are common on Finiens's Material; these comprise the "mortal fey", fey who for the most part have limited lifespans rather than traditional fey immortality, and whose mindsets are close enough to mortals that they can interact semi-normally, though the fey will always seem a little "off" in their behaviors, taboos, morality, and ethics in the eyes of mortals. The other "mortal fey" include Gnomes (who have retained their Fey typing in Finiens) and the Leanaí-Dubh, both of which will be discussed in the future, and an honorary recognition of such toward the Elves and the Ael-Vari, who likewise will be touched on later, despite them having lost much of their Fey nature and become Humanoid creatures over the millennia. There are also other "mortal fey" races who are present but less common; these are the published such fey races such as the Killoren and the Gathlain.