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