Peter Simple is Master Slender's not-so-bright—ahem, simple—servant. He spends most of his time running errands and delivering messages for Slender and his pals.
Robin is Falstaff's errand boy. He acts as a go-between for Falstaff and the "merry wives." Even though he works for Falstaff, he's a tad disloyal to his boss and helps the wives dupe Falstaff.
Rugby is Doctor Caius's servant. he spends a lot of time tagging along wherever Caius goes and sometimes helps Mistress Quickly.