The Characters of Arcadia all are a specific type of fae race or a kith. Some of these are carried over from Changeling, but there are quite a few that are unique to Arcadia (like the Dragonkin, Mechorg, Cog, or Army Ant), and some that are unique to Changeling (like Boggans), though they could have appeared in later Arcadia sets. Similar to the Court you belong to (Seelie or Unseelie) the kith of your Character can be as significant as you want it to be. In the role-playing game of Changeling, these are obviously important, and there is a rich background for every of the main kith that you can select. The main book describes them briefly, but there are also entire “Kithbooks” that detail the history, character, and main personalities of these kith. There is one for (K)nockers, one for Satyrs, one for Eshu, one for Pooka, one for Sluagh, one for Redcaps, and one for Trolls, that a player has impressively revised.
A few of the aquatic kith found in Arcadia, were further developed other White Wolf publications. Selkies were first detailed in Immortal Eyes: The Toybox, released in 1995 (see pp. 121-128), and in Immortal Eyes: Shadows on the Hill (pp. 78-85) . Mer(folk) were discussed in Blood-dimmed Tides, a World of Darkness sourcebook that was published in 1999, after Arcadia was released (see pp. 59-70). Triton’s resemble the Heiké Crabs discussed in that same book (p. 56). The Kokua are one of the Menehune, the kithain of Hawai’i, detailed in Immortal Eyes: Shadows on the Hill (see pp. 128-149, particularly 136-137).
Fianna, a race you can choose in King Ironheart’s Madness, is a tribe of Werewolf (or Garou). These are developed in White Wolf’s Werewolf: The Apocalypse, but are also briefly discussed in Changeling: The Dreaming, which is set in the same World of Darkness (see pp. 284-285). As the Changeling rule book (p. 264) states, “Werewolves are believed to be a race of changelings, and though they have left the company of their brethren, they are always welcome[d] back.”
Again, you don’t need to know all this background to enjoy Arcadia, but knowing a little of the background of your Character’s Kith will enhance your game.
The pages linked to below are from the Changeling: The Dreaming handbook. These were released by White Wolf for free as part of the The Changeling: The Dreaming Introductory Kit. They give a brief overview of the main kith in Changeling. Click on the image (the first page of the pdf) to download the entire file.
For a useful Changeling Kith Index, see here.
(Edited by Dark Yeoman, 18 December 2016)