one of his principal manifestations is Shesha, the cosmic snake. We must not be surprised, therefore, to find that Krishna, Vishnu’s human avatār and the conqueror of Kāliya, may be represented with the typical attributes of the serpent genii.
Zimmer argues that Vishnu’s identity with the cosmic serpent Shesha reveals that the divine conqueror and the Nāga he subdues share the same underlying substance, making the Nāga a manifestation of the supreme sustaining power rather than its mere adversary.
, Myths and Symbols in Indian Art and Civilization, 1946thesis