They'll never believe you, Henry! Mr. Pickles [barks] showed me the way of the Pavanatra Sluganotharats! -Agnes Gobbleblobber, Brain Download
Well it all started a long, long time ago, in the lost city of Pavanatra Sluganotharat. - Mr. Pickles, Season 3 Finale
Pavanatra Sluganotharat is an ancient civilisation from which Mr. Pickles heralds. Their existence is verbally attested twice in the series, and while their fate is unknown they shape the series' background lore.
Pavanatra Sluganotharat has a distinctive symbol, a sort of decagram composed of two overlying trigrams and a pentagram, the latter being topped by two horns. A central circle and unknown script are also seen in objects where this symbol is depicted.
From the flashback in the Season 3 Finale, we see that they are a vaguely nordic culture with roman architectural clues; the name itself is gibberish but superficially similar to Sanskrit. This, combined with its inhabittants having various skintones and the fact that its located in North America, suggests that it was somewhat multicultural.
Assuming the various arcane technologies Mr. Pickles has originated in it, it was a very technologically advanced society.
The Pavanatra Sluganotharats sacrificed dogs to the sun. This turned out to be their undoing since one of their rituals resulted in Mr. Pickles' birth, which is implied to have destroyed them.
Although Mr. Pickles is implied to have destroyed Pavanatra Sluganotharat, he apparently maintains their cultural ways, since Agnes Gobbleblobber claims that he showed her their ways.