From 1d4chan

Fiend is a catch-all name used to collectively refer to any and all of the malevolent entities that could hail from a game's equivalent of the Lower Planes. It is a nondiscriminatory term that encompasses both races such as Demon, Devil and Daemon (or other "Evil Outsiders") as a whole, and the myriad sub-varieties of each of those races.

Fiend was popularized as a catch-all thanks to the Dungeons & Dragons setting of Planescape; motivated on the meta-level by the dread Satanic Panic, in-universe, it was a polite term that planars came up with in order to avoid attracting angry "corrections" from either Tanar'ri or Baatezu, who both looked similar and absolutely despised being mistaken for each other, given their races were locked in the Blood War.