Catsclaw, that one is a devil rather than a demon.
Alzrius, this is a new take on an old theme.

Right now, I'm trying to nail down what we know about what demon lords appeared where, and all that, trying to be as complete as possible. I've also decided to go down everything on the list, one at a time.
First of all, I started with all the demon lords we know of who are not in the index at the back of FC1. The index was rather extensive and got just about everyone... but not quite everyone. Some of them were left off on purpose, some were left off by mistake, some existed but maybe had not yet been nailed down as demon lords, and some were yet to be created. So that yields the following list, which is as complete as I could get with the books I have:
Nql is the oldest of these forgotten demon lords. He was mentioned in the 1976 supplement Eldritch Wizardry, in the "Codex of the Infinite Planes" entry on page 43. The Wizard Tzook recorded some information, although much of it appears only in fragments: "... so knowing both the secret and the spell which would unlock the Way to this horde of the Demon Prince Nql ..." It's been speculated that he may not actually exist, or that "Nql" is possibly a fragment of another demon's name. But you know how demons are, we may never know the real truth!
Ereshkigel and Nergel were both included on the big list of demon lords on page 35 of the 1983 Monster Manual II, but I believe the designers of FC1 omitted them intentionally, most likely because they were named after real world deities instead of real world demons. A demon named Nergal appeared in Gary Gygax's Gord novels, as a prince of "unlife".
Arlgolcheir was mentioned in Dragon #116 in Ed Greenwood's article "Rogue Stones and Gemjumping" (page 55). He was said to have been desroyed by Laeral, with the aid of Khelben "Blackstaff" Arunsun and the mage Alduth of Neverwinter. He was further developed by Eric L Boyd in Dungeon #148 in the adventure "Wells of Darkness" as one of the powerful beings trapped in one of the wells.
Maelfesh was featured in Rose Estes' Greyhawk novels. I haven't read any of these books, so I really can't tell you anything useful about him.
Zanassu the Spider Demon was first included in Empires of the Sands, where he is said to rule over the evil creatures of Spider Swamp, and he is given a full stat block and description on page 61. He was later reported in the book Demihuman Deities as having been slain by the drow god Selvetarm.
Gorion was mentioned in Dragon #139 (Nov 1988) in the "Lords & Legends" feature on page 18. His evil had spread over the southern portion of a land called Sard, until he was banished by a band of adventurers.
Miska the Wolf-Spider and the Queen of Chaos were first mentioned in the second edition 1989 Dungeon Master's Guide by David Cook, under the "Rod of Seven Parts" entry on page 91, and the Book of Artifacts also on page 91, but they weren't fully developed until the Rod of Seven Parts boxed set adventure in 1996. Their backstory was further developed in the Age of Worms adventure path, and while they were both mentioned a few times in FC1, they were not included in the index in the back of the book.
Xazivort appeared in From the Ashes in 1992 on pages 91-96 of the Campaign Book in the adventure "The Sin Eater" by Carl Sargent. He is not to be confused with the xvart god, Raxivort, as he is a "powerful, unique tanar'ri" able to possess living beings; may not be an actual demon lord.
Lindyrm was mentioned in the Planescape adventure, The Deva Spark (1994) by Bill Slavicsek on pages 2-3 where it is revealed that the titular deva is seeking to uncover the demon lord's plans.
Kalphazor appeared as "A bitter old balor" in Dungeon #55 (Sept 1995), in the adventure "Umbra," page 19. I'm not sure if he was an actual demon lord, or if someone just mistakenly added him to the Wikipedia list.
Yrsillar the Lord of the Nothing appeared in Dragon #277 (Nov 2000), detailed fully and statted on page 164 in the "Rogues Gallery" feature by Paul Kemp.
Lascer, Lord of the Shadow Shoal, first appeared in Dragon 297 (July 2002) on page 27 in the article "Sentinels of the Shoal" by James Jacobs, and received a full description in Dragon #359 in the "Demonomicon: Apocrypha".
Azuvidexus the Ravenous Maw appeared on the Wizards' website in a feature by James Jacobs called "The Crawling Jungle":
Random Encounters: The Crawling Jungle He later appeared as an awakened tyrannosaur who had absorbed the demon lords's evil and intellect, in the Demonomicon in Dragon #357
Malcanthet first appeared in Dungeon #112 and #124 in the Maure Castle sequels, and was featured in a big way in the Demonomicon series and FC1, so her absence in the index must just be an editorial error. She could make a return in 4E if she were repurposed as something other than the Queen of the Succubi.
The Malgoth appeared in Dungeon #117 (Dec 2004) in the adventure "Touch of the Abyss", and was mentioned in FC1 p 125. This creature was destroyed by an alliance of demon lords, but now exists as a disembodied intelligence. Erik Mona identified the Malgoth as a demon lord.
Ubothar appeared on the Wizards' website in an adventure by James Jacobs called "The Thunder Below":
FREE Original Adventures: The Thunder Below (17th-level) He was later mentioned in the Demonomicon in Dragon #349
Kizarvidexus the Horned Duke appeared in Dungeon #124 (July 2005) on page in the adventure "The Whispering Cairn" by Erik Mona, and Dungeon #128, and was detailed more in the adventure "Wells of Darkness" in Dungeon #148. He slew one of the Wind Dukes of Aaqa, and was in turn slain by another of them.
Ebulon was actually mentioned in Fiendish Codex I on page 138, as a former general of Graz'zt and one of the prisoners in the Wells of Darkness, but was not included in the index.
Mu-Tahn Laa played an important role in the adventure "The Shattered Gates of Slaughtergarde" by David Noonan, first mentioned on page 3.
Veshvoriak was mentioned in Dragon #345 (July 2006) on page 31 in the Demonomicon of Iggwilv feature by James Jacobs as having been dumped in Kostchtchie's layer and having been kept alive, trapped in the ice. He was also mentioned in the Demonomicon book.
Chernovog the Green God was mentioned in the adventure Expedition to Castle Ravenloft (2006) by Burce Cordell and James Wyatt.
Vroth-Khun was mentioned as being banished from the Abyss and trapped in another realm, in the Demonomicon feature by James Jacobs in Dragon #349 (Nov 2006), page 31.
Xinivrae, an ancient foe of Malcanthet, and Sifkhu, a loumara lord described as being bonded with Malcanthet, were mentioned in the Demonomicon feature in Dragon #353 (March 2007) on page 27.
Zuregurex the Lord of the Drowned Dead was mentioned in the Demonomicon feature in Dragon #357 (July 2007), page 24
Cyndshyra of the Seven Torments, Felex'ja the Tiger King, Ixinix the Lord of Blackwater, Qij-na the Shattered, Rhindorz'zt the Black Prince, and Wejinhastala the Tempest allied with Ansitif to destroy the Malgoth, but they all assassinated or imprisoned each other; this is revealed in the article "Gazing Into the Abyss" by Eric Boyd in Dragon #357, page 67.
Rr'ikin'aka the King of Scorpions, and Ungulex were demon lords listed on page 66 as two of the several prisoners in the Wells of Darkness in the adventure of the same name by Eric Boyd in Dungeon #148 (July 2007). Also, the demon lord Voroxid is mentioned on page 81 as one of the demons whose true names are listed in a copy of the Demonomicon of Iggwilv.
Sertrous the Prince of Heretics received his own chapter in the book Elder Evils, beginning on page 112.
Zivorgian the Lady of Ripe Carrion, said to be the daughter of Pale Night, was mentioned in the Demonomicon feature in Dragon #361.
Codricuhn was the first brand-new demon lord to get his own feature in the Demonomicon column in Dungeon #168.
Rimmon was mentioned in the fourth edition's Monster Manual 3 on page 15 as having been destroyed in the destruction of the god Amoth.
Oublivae was introduced in the 2010 book Demonomicon, where her full description and statistics are given on page 130, which also states that she slew Ugoreth. Phraxas (who is apparently really just Anthraxus renamed) is likewise described in the book on page 134, having replaced Mydianchlarus (see also page 50); both were yugoloth lords in Planescape.