The Slugs are characters from the episode The Uncommon Cold. They are a group of intelligent, seemingly magical slugs who likely lived free in the Bayou until they were imprisoned by Big Bayou. As their imprisonment was a result of magic, they were unable to escape, and were forced to spend their days working hours on end in a dark cave, stuffing the snake's shed skins with leaves(thus making life-sized statues) and placing them around the bayou.
At some point, they managed to sneak into Big Bayou's house to peek at his magic book, but were only able to steal 'a smidge of magic', and thus couldn't free themselves. They were, however, able to conjure 'the uncommon cold', which allowed them to send a message to someone they hoped could help them. In the episode of the same name, this person happened to be Muriel. They also revealed that the cold could only be cured by magic.
Courage, eager to help Muriel, took her to the bayou, where they met the slugs. With Muriel unable to do much with her nasty could, Courage himself had to go steal the book from Big Bayou's house himself, leaving Muriel there with them(and her nail file).
When he returned, the apparent leader of the slugs(the one who sent the magic) politely allowed him to cure Muriel first. Before Courage could get to the spell that would free them, he was chased off by Big Bayou. However, Muriel found another way to break them of their imprisonment, using her nail file to break their chains. The smallest of the slugs then declared Muriel a hero, or 'big bayou Mama'. Likely, they returned to whatever they had been doing before Big Bayou after the episode ended.