Hags are ancient creatures, grandmotherly before there were children or grandchildren to mother. The first hags were the Night Hags, who were the children of Khyber (and are thus Fiends) who emerged at the beginning of creation. Since then, other forms of hag have emerged, primarily from Thelanis, playing the role of the wretched old woman (though there are also male hags, it's complicated). Native hags can also materialise on Eberron, in places where there are stories of such beings.
Some of the most famous hags are the Daughters of Sora Kell, who founded the nation of Droaam.