Mr. Popo shows up at the very end of the original Dragon Ball anime as Goku meets him shortly after defeating King Piccolo.

Despite Goku being able to defeat King Piccolo…. for some reason Mr. Popo is able to hold his own against Goku where he’s not hurt at all… which makes absolutely no sense.

But he introduces Goku (badly) to Kami and they revive Shenron and get Goku ready to fight Piccolo Jr. in four years time.