Squeaky Farms Brand Genuine Animal Milk was a rat milking company owned by Fat Tony.
Fat Tony made a deal with Mayor Quimby to sell milk to Springfield Elementary School, promising dog milk or better. However, he instead chose to milk rats with the belief that since people like rats, they would like their milk. Homer discovered this plot while visiting the Legitimate Businessman's Social Club as Quimby's bodyguard and convinced Quimby to put an end to the business. Fat Tony was arrested and the business was shut down. All of the milking rats were set free into Springfield.
Behind the laughter
The milking company and its ties to the Springfield Mafia is an indirect reference to how Al Capone during the Great Depression passed himself off as a humanitarian as a cover for his organized crime lifestyle, with one front being the distribution of milk to schoolchildren.