I will let someone with real skill and knowledge to do a real write up about the Unicorn Clan. Meanwhile, much like Toshimo-san, I will add some stuff based on old AEG lore. FFG may have ignored or changed some of the stuff though.
After Fu Leng had his Kami ass kicked by the human Thunders, Kami Shinjo thought that there could be more threats outside of Rokugan and it would be a good idea to explore. Knowledge is power and all that. Before departing with her whole clan, she left behind those who did not fancy the trip, who later became the Fox Clan, so they would chronicle events in Rokugan for later reading by the Clan upon their return from the exploration.
During their travels through the Burning Sands, the clan fought and later accepted the Ujik-Hai (later became the Moto), they got gaijin horses (our regular horses, not ponies like in Rokugan), Shinjo had scions with a real unicorn, the Qolat inflitrated the Clan through the Ujik-Hai, and while trying to get back to Rokugan after many more adventures, Shinjo was lost to the Lying Darkness while she sacrified herself to give her Clan time to scape.
One of Shinjo's children took care of the Clan and renamed it to Unicorn. Another of her children, that had the physical form of a unicorn, breed with the gaijin horses and produced the Utaku Steeds (warhorse like). When the Unicorn clan reached Rokugan through Crab lands, the Crab scouts had reports about them and had mounted a defense that... did not work. The Unicorn just run through them and just did not stop. The Unicorn Clan was finally accepted as the returned Ki-Rin when they gave the Crane the present that Kami Doji had given her sister Shinjo before they had departed.
- The Unicorn Clan has closer ties to the lower classes due to their experiences in the Burning Sands, where their survival needed them to work together. Also, Kami Shinjo's virtue was compassion and even have a law by Shinjo herself about not being allowed to kill those of their own kin.
- With them, the Unicorn brought many gaijin habits to Rokugan. From eating meat or using leather, gaijin weapony like scimitars or tools like spyglasses, to more simple things like not sharing bathing rooms between men and women.
- Rokugani clans tend to consider the Unicorn not honorable for not following their ancestor's traditions but they actually do. Just that those traditions are those who allowed them to survive in the hostile Burning Sands.
- The Fox Clan did not rejoin the Unicorn but shared with them their library.
- The mongol thing/theme is not really present as a main thing. Using a scimitar instead of a katana, for example, is not the norm by any means or common.
- The Unicorn are without a task within the Empire as they already fullfilled theirs of exploring outside Rokugan. Later they would be given the role of magistrates (once Second Day of Thunder passed) and when the Moto became the bosses, it was about mongol super army. I prefer magistrates and Shinjo in charge. But before the Scorpion Clan Coup, the Unicorn had no official duty like the Crab. Too busy trying to get used to Rokugan and Rokugan getting used to them?
- The Unicorn still keeps (secret) trade with the Burning Sands. That includes some holdings and members living there (like more Moto).
Steemed (Rokugani) Families of the Unicorn Clan
- Shinjo - Main family of the Clan. Much like their Kami, they like to roam and explore. Focus in bushi, scouts and magistrates.
- Utaku - This matriarchal family is of unquestionable honor. Their women join the dreaded Shiotome, charging silently on their powerful Utaku Steeds, while their men are in charge of taking care of their precious horses or alternatively join an infantry school or one of the other family schools, as only the Utaku women (with a few expections) are allowed to ride the Utaku Steeds.
- Ide - The Ide are the diplomats of the Clan. Having to deal with different gaijin cultures, having expert diplomats to be able to deal with them was fundamental to survive in the Burning Sands, be it to seek allies, commerce or just to avoid conflict. Focus on diplomats, merchants and magistrates.
- Iuchi - The shugenja family of the Clan. Based around water magic and its mobility, the Iuchi are also very curious and, from their travels through the Burning Sands, learned about a magic called Meishodo. This Name Magic does not involve calling upon the kami.
- Moto - Descended from the Ujik-Hai from the Burning Sands, these fierce warriors had suffered a lot. Trying to help the Crab, Moto Tsume lead an attack on the Shadowlands that did no only fail but ended with them corrupted and lost to the Shadowlands. Such dishonor still haunts the Moto, who are usually retired from the public eye. If you were to describe them a Hide berserker on a Horse, it would be close.
- Horiuchi - During an incident in which the Shinjo Champion's scions where in danger, their nanny, Iuchi Shoan, protected them using a moat. As a reward, she got her own family with Hori added to the Iuchi (Hori meaning moat). Horiuchi Shoan is truly gentle and probaly the first and last of her family, her only romantic interest being a Crane that is not interested. (Probably to be eliminated/ignored by FFG? )