Veles was God of forests and wildlife, later and God of agriculture. Some Slavs consider Him as protector of poets and Nav - realm of dead. He was represented either as shepherd, horned God, who had bear skin on His back, or as large horned serpent.
His arch-enemy was Perun who battled Him whenever He wanted to climb upon World Tree. But He was always defeated and that defeat brought rain and thunder.
Despite their role in Slavic folklore, Slavs would make 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.The most controversial writing Book of Veles is named after Him.