In Greek mythology and religion, the term daemon was ubiquitous, referring to supernatural agents or intelligences, lower in rank than a god and holding a middle place between gods and humans, such as the Corybantes, Curetes, Dactyls, Satyrs and Sileni. Spirits of forests, rivers, glades and mountains, as well as cities presided over public and family life and were also referred to as daemons. Daemons could be either good or evil, but even good ones were believed to be capable of evil acts if angered by humans.
Daemons could also be ministering spirits, god like beings, souls of dead persons, or familiars (companion or helping spirits that take on animal forms). Gernerally they were considered by the Greeks to be protective and attending spirits much like guardian angels or Plotinus's notion of tutelary spirits. In addition, gods themselves are invoked as daemons in certain texts.
"Belief in daemons dates to ancient Mesopotamia. The Babylonians had an elaborate daemonolgy, in which daemons were organized in armies and hierarchies, and like angels had specific duties."
~ Georg Luck. Arcana Mundi: Magic and the Occult in the Greek and RomanWorlds.