D&D Monsters: Devils

In regular English, the term 'devil', when not applied specifically to Satan, is essentially synonymous with 'demon'. Whereas the word 'demon' originally had a more benign meaning, 'devil' has always meant an evil entity, and now typically means one that is specifically part of the Christian mythos even if the general concept exists in other religions, too. 

In D&D, however, devils are distinct from demons, making up the organised legions of Hell rather than being rampaging creatures of malevolent chaos. In 1E, six main types exist, although other common ones have been added since, all fitting within a defined hierarchy where weaker devils can (with difficulty) be promoted to higher ranks at the whims of those even higher up the chain. Compared with the demons, these six standard types are more likely to owe their origins to myth or at least to traditional depictions of such beings, rather than just being odd combinations of animal parts. 

D&D Monsters: Erinyes

The Erinyes originate in Greek myth, where they are goddesses of vengeance cursing kinslayers, oathbreakers, and the like; they are probably better known under their Roman name of "Furies". These original versions appeared as ugly women, typically with snakes entwined in their hair and wrapped around their bodies and limbs. Sometimes they had wings, sometimes not, and later depictions of them are similarly varied.

Some versions of the myths state that there are only three Erinyes, but others are much vaguer about the numbers. Notably, the three named Erinyes stand guard over the City of Dis surrounding the Sixth Circle of Hell in Dante's Inferno. It may be this that inspired their adoption as a type of "devil" in D&D, although they (and Dis) are moved to the Second Circle in the 1E Monster Manual, befitting their status as the weakest of the true devils in that edition. Something of a demotion from their mythic origins, then.