Veles, or Volos, is the name of the pre-Christian Slavic God of Cattle, who in addition to his role as protector of domestic animals, was also the God of the Underworld and the bitter enemy of Perun, the Slavic God of Thunder.

Veles in Slavic Mythology

The earliest reference to Veles is in the Rus-Byzantine Treaty of 971, in which the signers must swear by Veles' name. Violators of the treaty are warned of a menacing punishment: they will be killed by their own weapons and become "yellow as gold," which some scholars have interpreted as "cursed with a disease." If so, that would imply a connection to the Vedic god Varuna, also a cattle god who could send diseases to punish miscreants. 

Veles is associated with a wide variety of powers and protectors: he is associated with poetry and wisdom, the lord of the waters (oceans, seas, ships, and whirlpools). He is both the hunter and protector of cattle and the lord of the underworld, a reflection of the Indo-European concept of the netherworld as a pasture. He is also related to an ancient Slavic cult of the deceased soul; the ancient Lithuanian term "welis" means "dead" and "welci" means "dead souls." 

Appearance and Reputation 

Although few images exist, Veles is generally portrayed as a bald human man, sometimes with bull horns on his head. In the epic creation battle between Velos and Perun, however, Veles is a serpent or dragon lying in a nest of black wool or on a black fleece beneath the World Tree; some scholars have suggested he was a shape-shifter.

In addition to domestic horses, cows, goats, and sheep, Veles is associated with wolves, reptiles, and black birds (ravens and crows). 

Cosmic Battle Between Perun and Veles

The best-known myth of Veles is found in several versions, or fragments of versions, from the various cultures claiming descent from the Kievan Rus'. The tale is a creation myth, in which Veles abducts Mokosh (the Goddess of Summer and consort of Perun, God of Thunder). Perun and his enemy battle for the universe under a huge oak, Perun's holy tree, similar to both Greek and Norse (Yggdrasil) mythologies. The battle is won by Perun, and afterward, the waters of the world are set free and flowing.  

Separating the Human and Nether Worlds

A second creation myth associated with Veles is the formation of the boundary between the underworld and the human world, a result of a treaty forged between Veles and a shepherd/magician. 

In the treaty, the unnamed shepherd pledges to sacrifice his best cow to Veles and keep many prohibitions. Then he divides the human world from the wild underworld led by Veles, which is either a furrow plowed by Veles himself or a groove across the road carved by the shepherd with a knife which the evil powers cannot cross. 

Post-Christian Changes

There are many possibly recognizable vestiges of Veles remaining in the Slavic mythology after Vladimir the Great brought Christianity to the Rus' in 988. Velia remains a feast of the dead in old Lithuanian, celebrating the border between the world of the living and the world of the dead, with Veles operating as a role of guiding souls to the underworld. 

The battle between Perun (Ilija Muromets or St. Elias) and Veles (Selevkiy) is found in many different forms, but in later stories, instead of gods, they are complementary figures separated from one another by a furrow plowed by Christ, who converts them. Veles is also likely represented by St. Vlasii, depicted in Russian iconography as surrounded by sheep, cows, and goats.

Sources 

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