The rat boys from my campaign.
They were recruited by my player when they attacked the city he (my player) was currently using as a hub. Though they weren't a completely random encounter, they're that not far from.
What I had planned for that session was that the city would be overrun by rats led by a single wererat, maybe two if the encounter turned out to be too easy, sent there by the hags to wreak some havoc. It didn't take long until the player found the wererat playing a flute to summon the rats, and promptly tried to kill him. The flute-rat tried to escape, but was cornered by the player on one side, and by a pit of black writhing tentacles on the other (Hunger of Hadar is my player's favourite spell). He tried in vain to summon more swarms of giant rats, and was eventually rescued to some extent by the other wererat, but in the end they were overpowered. My player, however, gave them the option to either betray the hags they were sent there by and start working for him instead, or be slain.
The rat boys decided to join him almost without hesitation, and I had to come up with names, looks, and basic personality traits on the spot. The flute-player was named John, and described to look like a bard, though he wasn't proficient in playing any instrument. And the other one got the name Jake, and looked like a scarecrow.
Their personalities are more developed now - John being a happy-go-lucky moron, pretty much. And raised by a Fire Giant Cult before he was attempted kidnapped by Melody Toestealer the Annis Hag, but instead found common ground with her and thus became her first brute. Jake is a hard working farmboy who became Melody's second brute against his own will.