The Mild Ones are a biker gang made up entirely of elderly citizens (besides Squidward), who are mistakenly thought by SpongeBob to be the Wild Ones. The group only appears in the episode, "Born to be Wild."
SpongeBob is in Jellyfish Fields when he sees them, not knowing their age, thinking they are the Wild Ones (who are known as the most ferocious, bloodthirsty biker gang under the sea). SpongeBob decides to make his own biker gang, the Bikini Bottom Bad Boys, to try and defend Bikini Bottom. SpongeBob finds out later that he mistook the "M" for a "W."