Origins
The Xiun were tribal mannish horse lords until being united by Invidia the Maiden. During a meeting with Galiphon the Gilt and Koseth the White Handed, Milisai herself came down. Being in that close a proximity to a Diva changed the Xiun into the birdfolk we know today. Their horse also turned into winged creature known as griffons.
Characteristics
Samurai kenku.
Generating Xuin Characters
The race used for portraying the Xiun is generally kenku as described in the MM. Kenku got a buff in a recent Dragon and may actually be viable. Githzerai out of the PH3 or Deva out of the PH2 actually look like pretty good alternatives. Hengeyokai from Dragon 404 also looks good, and it's Oriental themed!