Veles

Veles was the Slavic God of forests and wildlife, and later God of agriculture. Some Slavs consider Him as protector of poets and Nav - realm of dead.

The most controversial writing Book of Veles is named after him.

Battle between Perun and Veles
His arch-enemy was Perun who battled Him whenever He wanted to climb upon World Tree (similar to the Norse Nidhogg). As this would cause the human realm (Yav) to dry out, Perun would battle Veles. Veles was always defeated and that defeat brought rain and thunder on the human realm.

Oath of Peace
Despite their role in Slavic folklore, Slavs would use Veles as an oath of peace between them and their enemies. The most known example was when Slavs made peace with Byzantine Empire (Greeks):

"And they made an oath with their weapons, their God Perun and Veles, the cattle God and created peace in year of 907."

Appearance
He was represented either as shepherd with horns and had a bear skin on His back, or as large horned serpent.