Paul Dana's Time Travelers of Oz gives an origin story for the Herkus and their slaves, explaining that Mrs. Yoop turned some of the inhabitants of the city into giants, and the zosozo was invented to help the ordinary people in dealing with these newly-made giants. If that's the case, however, I have to wonder why the smaller Herkus would treat their old neighbors so abusively. I'd propose that they forgot about the origins over the generations, but the story has Vig living in Herku at the time of Mrs. Yoop's invasion.
I'm kind of embarrassed to admit that it took me a while to realize the obvious derivation of "Herku" from "Hercules." The name of the city's ruler might well be a play on "vigor," with his title being "czar" (as in the the Emperor of Russia ) plus "over" (to fit with his reputation as the World's Most Mighty Ruler). But then, it's not like I have official word from L. Frank Baum on any of those.
Tomorrow, we'll take a look at the most infamous Herku of all.
 The Lost Princess of Oz was published in 1917, the same year in which the last Tsar, Nicholas II, abdicated.