Lost Tribe

You've got an ethnic group, pretty much hunted to extinction. But they're not all dead. Enter these guys, the lost tribe. Goes hand in hand with Exclusively Evil, Always Lawful Good, Schizo-Tech, and Planet of Hats. Also seen with Last of His Kind.

Examples of Lost Tribe include:

Anime & Manga

The B-movie Mexican Blow has Indians from north of the Rio Grande who migrated to southern Mexico to escape the white man and speak English so they won't be tricked by the white man again. All the Indians are white, but the main character is a Mighty Whitey too.

At Play in the Fields of the Lord has theories that the Niaruna are the Lost Tribes of Israel or a lost tribe of Sioux.
The Star Wars Expanded Universe has the Lost Tribe of the Sith. Also the Jedi in Dark Empire. And the Dathomiri. Yeah, lot of lost tribes in Star Wars.
Kushiel's Legacy has the main character of the first trilogy run into the Tribe of Dan, one of the Lost Tribes of Israel, guarding the Ark of the Covenant down around what in our world is Lake Victoria.

The titular lost tribe in the Stargate Atlantis episode "The Lost Tribe".
- The team also finds a group of Ancients fleeing their long-past war with the Wraith at relativistic speeds midway through Season 3.

The lost tribes of Israel are the Trope Namer. Either in The Bible or The Book of Mormon. Mormons still tell American Indians they are the lost tribes.
In New Age beliefs, American Indians are aliens.

Chrono Trigger has a lost tribe of Mystics, and survivors of the Kingdom Of Zeal.
- Chrono Cross has survivors of other timelines.
Final Fantasy likes to use this trope:
- Subverted by the Lunarians in Final Fantasy IV
- Played ridiculously straight with the Espers in Final Fantasy VI.
- Venat's people in Final Fantasy XII.
Legend of Dragoon has the last surviving dragons.
Uzo and Muzo are the last survivors of the Mt. Gokuho clan in the game "Kingdom of Paradise".

The Nazis considered the Sioux, Japanese, and Tibetans to be Aryans. Only the Sioux rejected the label.
- Actually, they only considered the Japanese "honorary Aryans". It was fully expected that when they didn't need each other anymore, the two nations of Proud Warrior Race Guys would go right back to considering the other absolutely inferior.
Lost tribes of North American Indians are found all the time. They live like modern people and are really not all that different.
Perhaps the best example of this trope in real life is the Sentinalese tribe, which may very well be the most isolated group of indigenous people on earth. They live on North Sentinel Island in the Indian Ocean, and violently reject all attempts at contact, shooting at all outsiders with arrows. Because of that, we know next to nothing about them, including their customs, language, and how they live. The last attempt at contact was made in the mid 1990's, and since then, they've been left alone, and it's very unlikely that will change anytime soon.
The Other Wiki has an article on the subject.