I think you've got the wrong impression here, Kuruni. The former demon lord isn't a collective term for ALL the former holders of that office. This article title is "Former demon lord", not "Former demon lordS". There were previous ones before this one, but when the Encyclopedia says "former demon lord", it means JUST the one before the succubus one, not every other one as well. So what you've written is misleading, imo. Goddess Ilias (talk) 10:03, August 31, 2015 (UTC)
It's more of translator's choice. Japanese text stick to 旧魔王 without indication of singular to plural, but it doesn't indicate to be specific entity either, and at least Demon translated profile use former demon lords.
"JUST the one before the succubus one" Now, what make it different from the one before that? We never have information how each of them were different from another, we not even know which race each of them were. It is background information at best.
"This article title is "Former demon lord", not "Former demon lordS"" Of course, article about concept doesn't use plural title. Wikipedia's article on monarch is Monarch, nor Monarchs.--My girlfriend is a loli. 16:29, August 31, 2015 (UTC)