So speaking of talking about UPG for PBP...
I've only written about Veles briefly on here, but I can't let 'V' go by without saying something. Veles, if you don't know, is a Slavic deity with a pretty interesting portfolio. He's most notable as the opposite number of the Slavic thunder god Perun. Pretty much every Indo-European pantheon has one of these pairs, a thunder god and an underworld serpent god that battle.
My original draw to Veles was actually an outgrowth of my interest in the Norse pantheon - comparing Thor and Perun led me naturally to compare Jormungand and Veles. Since there's very little on Jormungand in the lore, I was drawn to Veles instead; it was much easier to form a relationship with him.
Plenty of people are use the term "squishy polytheism" lately, and this is one area where squishiness comes into play for me. I definitely see Veles and Jormungand as different beings who come from the same source.
Veles's battle with Perun is one with a foregone conclusion. Every year, Perun sends him back under the earth. It's one of those myths that makes you stop and think about "myth as metaphor" vs "literal myth". There's no good reason to keep doing the same thing when you lose every year, after all, so there's got to be something more going on there.
The victory or loss is not really the point, as far as I can tell. Whether the cycle is yearly or comes around once every Ragnarok, it's a reminder that destruction is necessary for creation. Defeat for the serpent isn't really defeat, and the serpent is victorious just as often as the thunder is. The difference is that we only celebrate the thunderer's victory.