“When a monster is associated with an anthropomorphic deity, it operates in the same field of action or part of nature as that of the deity.
Whereas the deity functions in the entire domain of his or her rule, the monster’s activity is limited to only part of the god’s realm. Thus, a monster that is associated with a deity as its attribute creature represents part of the divine nature or a particular aspect of the divine function of the god.
Wiggermann observes that after a developmental period, during which Mesopotamian gods and monsters evolved, they eventually settled into “complementary” opposition in which “the gods represent the lawfully ordered cosmos, monsters represent what threatens it, the unpredictable.”
(Frans A. M. Wiggermann, “Some Demons of Time and Their Functions in Mesopotamian Iconography,” in Die Welt der Götterbilder (ed. Hermann Spieckermann and Brigitte Groneberg; Beihefte zur Zeitschrift für die alttestamentliche Wissenschaft 376; Berlin: Walter de Gruyter, 2007).
The 1992 illustrated dictionary written by Jeremy A. Black and Anthony Green, Gods, Demons and Symbols of Ancient Mesopotamia, has provided an initial launching point for dealing with the maze of interrelated deities, demons, and composite creatures of ancient Mesopotamia.
(Jeremy A. Black and Anthony Green, Gods, Demons and Symbols of Ancient Mesopotamia (illustrated by Tessa Richards); Austin: University of Texas Press, 1992).
While the work is far from exhaustive and does not provide references for its sources, it has proven to be a valuable guide through the daunting complexities of the topic.
Constance Ellen Gane, Composite Beings in Neo-Babylonian Art, Doctoral Dissertation, University of California at Berkeley, 2012, pp. 3-4.