My books are published by Perkunas Press, and every now and then someone asks about the origin of the name “Perkunas”.
In the picture above, he's the guy in the middle: Perkunas, the Baltic god of, well, many things. In this instance, he represents maturity and power, vs age and death on the left, and youth and fertility on the right.
More commonly, Perkunas is shown with a thunderbolt. He is the champion of good, feared by all evil spirits. He rights wrongs, and upholds the balance of the world.
His clear counterpart in the Nordic countries is Thor, and there is a long and tangled relationship between them. The details are buried in lost history and intriguingly suggestive Indo-European etymology.
The root for “oak” in Indo-European is *perkṷu (the “*” indicates a reconstructed form, IE being unattested except by its daughter languages). We see that word reflected in Latin: quercus (oak), and there is a general association of oak trees and lightning in Indo-European mythologies. Gods of oak trees are gods of thunder, and they wield thunderbolts (with emphasis on the pounding aspect of lightning rather than the flash).
In Norse mythology, there are two tantalizingly references to obscure gods: Fjörgyn and Fjörgynn. The former (female) is mentioned briefly as the wife of Odin and mother of Thor, and the second (male) is mentioned as a byname for Odin. Etymologically, Fjörgyn is related to the same Indo-European word and seems to be another form of Perkunas. Lots of speculation exists about what this means in the relationship of the Norse with their Baltic neighbors.