Hiya!
Well, upon thinking about stories, myths, etc, [MENTION=5834]Celtavian[/MENTION] and [MENTION=996]Tony Vargas[/MENTION] are correct. There are 'lots' of stories about single 'boss monster' fights in the end, as they mentioned. I guess what I was thinking about was D&D and AD&D 1e/2e adventure modules. Of the official published modules, I can't really come up with any where the PC's faced off against a single boss-monster type. Hmmm....
**SPOILERS BELOW**
**
**
**
**
The module B4, The Lost City ...the original one had a very bottom level where the PC's could face off against "Zargon" (iirc), who was a unique monster. A case might be made for Oonga from the
Isle of the Ape module...but I can't remember if there are any of those other "little oonga-apes" around him. Oh, maybe the red dragon in the dungeon/cave in another Basic D&D module,
Horror on the Hill. Hmmm....
Lost Caverns of Tsojacanth has potential for a face off against the vampire chick...Derlena? Drelna? Something like that.
But that's all I can think of at the moment. Maybe there's more. I'm not including ones where the 'boss monster' is very likely to summon or otherwise 'have minions available'. I guess I'll have to concede there there are more adventures with "boss monsters" in them than I had originally though.
Er...however...

...I'd also like to point out that most of the boss-monsters I listed and can think of are all unique individuals or creatures. So there is still no real "solo" monster category in 5e, from what I can see. There are monsters that can fit that bill and hold there own, as I said, but I'd wager that to make them a "true, boss-monster", the DM is going to modify them to suit the story. So doing up an analysis of X PC's vs. Y Solo Monsters isn't going to work just from that standpoint.
^_^
Paul L. Ming