Rudd on the old Eskimo/eunuch riddle

Found a joke in the back matter of Niall Rudd’s translation of Horace’s Satires:

105. The Latin is “est inter Tanain quiddam socerumque Viselli” – “there is a point between Tanais and the father-in-law of Visellius.” The ancient commentator Porphyrion (third century A.D.) says that Tanais was a eunuch and that Visellius’ father-in-law represented the opposite extreme. This is not a particularly reliable or informative comment, but we have nothing else to go on, and so I have used it as the basis of my translation, which refers to the riddle: “What is the difference between an Eskimo and a eunuch? Answer: One is a frigid midget with a rigid digit; the other is a massive vassal with a passive tassel.”
—Niall Rudd (trans.), Horace: Satires and Epistles, Persius: Satires (rev. ed., 1997; my emphasis)