Dan the Fly Man has three episode appearances. He only speaks in his first appearance, in Frog's Best Friend. Here he works for Maggot Farms fly delivery service (hence the "M" on his hat), and delivers a cage full of flies to the Bigheads' house and then gets attacked by Earl the dog who bites his hiney and also topples and wrecks his van. The last we see of poor Dan is when Mr. Bighead is taking out the trash and sees Earl chewing on a tire; a sign reading "Help!" pops up from behind the overturned van.
Dan hears something in the Bigheads' salmon bushes, and sees Earl's familiar punk rocker hairdo sliding along the top of them, like a shark fin, and runs for his van. He hears a growl behind him, and whirls to discover...Spunky! He playfully admonishes the little dog for frightening him...and is then promptly attacked by Earl.
He later participates in two of the musical numbers in the episode Zanzibar!: "Spring Cleaning" and the reprise of "You Can't Fight City Hall" respectively. He is also one of the two characters who carry Rocko on their shoulders out of the Conglom-O boardroom at the end when Rocko convinces Mr. Dupette to recycle. (In a coloring error, Dan is teal instead of purple in this episode.)
Dan's third and final appearance was in Ed Good, Rocko Bad, where there is an advertisement for a TV show called "Stray Dogs." When the announcer is explaining how the dogs like to "bite the mailman just to hear 'im squeal," we see a dog doing exactly that to Dan. Either this is Dan and he quit his job at Maggot Farms, or they just recycled the character model. In any event he wears the same uniform with the stylized "M" on the hat. Another possibility is that, since "Stray Dogs" is a show-within-the-show, Dan could've taken up acting.
Dan the Fly Man is a fairly interesting character. On the surface, he's just depicted as a "luckless delivery guy" type character who gets victimized by dogs..., however the irony is that he is himself a dog. Meaning that in two out of his three appearances Dan, an anthro dog, is attacked by non-anthro dogs.