Okay, this is confusing. Buckle in.

Á-sàg, literally meaning "disorder", is a specific Sumerian supernatural entity ("demon" isn't a bad approximation) appearing in the text Lugal-e ud me-lám-bi nir-ğál, or Ninurta's Exploits, dating from the late 3rd millennium BCE onwards. Á-sàg was born of the union of An and Ki (a primitive earth goddess), which puts him in the class of demons known as sebittu, "group of seven" who were tasked with assisting Nergal, god of the underworld, with the transportation of dead souls. This seems to overlap with the function and number of the galla (Akkadian gallú), underworld demons that appear in magical incantations. Udug (Akkadian utukku) is a broader general term for spiritual entities (be they beneficent, deleterious, or neutral) that encapsulated demons of several types. In later Akkadian texts (incantations and disease lists), á-sàg lends his name to the asakku, a more generic class of entities that were considered to be responsible for head fevers and death from illness.

The important thing to keep in mind is that we're dealing with a huge mish-mash of beliefs and supernatural traditions spanning two languages, thousands of square miles, hundreds of distinct cults, and two thousand or more years of variation. These categories and appellations weren't decided upon and set in stone by some regulatory commission for all time; they represent a constant flux of folk beliefs and mythological interpretations, and we are only approaching it through partial reconstruction via whatever texts happen to have survived by chance.