"Nae king! Nae quin! Nae laird! Nae master! We wilna be fooled again!"
The Nac mac Feegle (also known as the Pictsies) are a race of savage, blue-skinned (from all the wold and tattoos they wear) gnomes (technically a sort of fairy) from the Discworld. They do a lot of running around shouting in Scots and stealing livestock, along with anything else that isn't nailed down (and if they can find a crowbar, they'll steal everything that can be pried loose). Their three favorite activities, in no particular order, are drinking, fighting, and stealing. Imagine a Smurf with the brain of an ancient Scottish tribesman (ala Braveheart), and you'll have a rough idea of what a typical Nac mac Feegle looks and acts like.
Speaking of Smurfs, another similarity the Feegles share is the gender division: almost all Feegles are male, with the exception of their leader, the kelda. A kelda can have hundreds of sons in her lifetime, but will usually only have one daughter, who will leave home when she comes of age to become kelda of another clan of Feegles. Almost all Feegle males are impulsive and not too bright, so the only Feegle with any brains around besides the kelda and her daughter is the Feegle war poet, or gonnagle. The gonnagles are bards who travel between clans spreading stories and legends. They are great musicians, but the high-pitched mousepipes they play can be a little hard on the ears. They are also infamous for stunning the enemy with astoundingly bad poetry.
The Nac mac Feegle originally hailed from Fairyland, but were thrown out by the Queen of the Elves for rebelling against her cruel orders (or for being drunk and disorderly, depending on who you ask). The Feegles fear nothing (with the possible exception of lawyers) since part of their "religion" involves the assumption that they're already dead, and are in heaven, with plenty of beautiful weather, good food, and things to steal and fight. Any Feegles who "die" are assumed to have been reincarnated. They also believe in the mystic powers of the written word, especially the sort of written words that can be used against them in a court of law.
They appear in the novels Carpe Jugulum, The Wee Free Men, A Hat Full of Sky, Wintersmith, and I Shall Wear Midnight.