Again I am ONLY talking about the 90s Anime version, since this theory is based exclusively on an almost never mentioned plot in what seems like an filler episode exclusive to that series.   This would be the Season 1 episode 40 "The Legendary Lake Yokai The Bond of Usagi's Family" ("Last Resort" DiC version)

That's the episode where the Tsukino family visits the hot springs where Ikuko recounts the "Legend of the Lovers" to Usagi when she sees Usagi standing in front of a statue of a loving couple.  I'll just quote Ikuko's story from the Viz dub:

"A long long time ago a young man fell in love with an angel who descended from the heavens.  Another woman who loved the young man was angry that he didn't return her feelings.  Consumed by hate and jealousy.  This woman turned into a monster and began attacking the villagers.  The angel and the young man used the power of their love to seal the monster away in the lake.  But having used up all their strength the two died and ascended to heaven."

This legend of the lovers obviously drew a strong parallel to the Beryl/Serenity/Endymion situation.  And I think many probably brushed it off an assumed the story was talking about Serenity/Endymion.  However there are problems with that theory.  Not the least of which is the ending where the two lovers apparently die and their spirits ascend into the heavens.   That would seemingly rule out this being a story about Princess Serenity/Endymion (or even Queen Serenity/Princess Serenity's Father).  I mean unless Queen Serenity brought her daughter and future son-in-law back from the dead twice...  Now I guess you could also maybe assume that the people were incorrect about their dying, however the flashback scene we see of the two bathed in a golden light ascending into the sky is indeed identical to what happens to the spirit of the jealous woman once she is healed at the end of the episode and her spirit is allowed to move on in that fashion.  So the lovers actually dying thing seems pretty convincing to me...

Next of course is the fact that the appearance of the two young lovers is completely different from Serenity/Endymion.  Starting with the hair color where the woman in the past clearly has dark hair and not blonde/silver hair.   The hairstyles of the two are also completely different.  The woman wore her hair long and straight in the back other than having her bangs looped in two bows which were only visible from the front.  While the guy also had longer hair and actually tied it up in buns of sorts.  Really from the back his hairstyle was strikingly similar looking to the way Usagi wore her hair in this episode especially when she was in the hot-springs.  (Which could explains why the yokai was drawn to focus on the buns on Usagi's head. From a distance only seeing the back of her head she probably recognized the hairstyle buns as similar to that of the man she loved!.  Note that the man's "angel" lover did not have buns visible from behind.)


Finally there is the fact that when the man and woman use their power to seal away the monster the energy glow that surrounds them and seals the monster is a mostly a pure white one with a little tint of yellow (not gold) at the edges instead of a brilliant pink one like there is every time the Silver Crystal is used.  (Even when Usagai/Mamoru summoned the Spiral Heart Moon Rod with the power of their love everything was still bathed in a pink light...yeah I know that would not happen until seasons later so it technically has no bearing here still felt worth mentioning though as part of the larger continuity.)  White light is not really Usagi/Serenity's thing.  The only two exceptions I can think of are first the white energy when Endymion touched the star locket and was cured of his brainwashing he was briefly filled with white light from that item.  But that energy was pure white without any tints, and it was not really an attack like this.  Clearly it just came from that item and was not even tapping into their life forces that they could have been completely used up like the story goes?  The second was when Usagi was in her final pure angel form the energy that poured from her and shattered Galaxia's sword was purest white.



Now I guess you could visually interpret all this as just being Usagi making up how things looked as the story is told (except the hairstyles because of the statues having them), however the monster we see is exactly like the lake monster she will fight later in the episode.  So to me that indicates these are more than that.  Could they be her memory?  I guess although she doesn't act like there is any recognition of this.  Also her remembering then still leaves the above points to be explained.  I mean I guess maybe Serenity could have used something like the disguise pen, but that doesn't explain Endymion's different enough appearance, the white sealing energy, or you know them both apparently dying...

You could write it off as just being about any other Silver Millennium girl (While technically the girl could have come from any of the planets, I don't think the manga had even yet established that they were all inhabited at the time?  So more than likely she would have been from Moon...but I guess there could still have been a possibility of her from being from somewhere else if you need an explanation for how she had powers.) and Earth man...except twice the jealous woman from the story shows a spark of recognition towards Endymion.  The first time is when he first summons her.  She rushes to attack him in her monster form but halts when she gets close.  Her eyes flicker as she looks at his face, and then she starts her "Give Him Back!" chant.  The second time is after she is healed and ascending into the afterlife the woman sees him at a distance and seems to recognize him yet again.  So uh did Endymion fall in love with a dark haired girl from the Silver Millennium before he met Princess Serenity?   I would certainly hope not...plus the whole pesky dying thing.   So then why does this monster/woman recognize him?

Thus the only thing I can come up with is what if it wasn't Endymion but someone that looked like him such as his father whom the story was about?  I mean after all in anime/manga/cartoon kids often become the spitting image or very close to it of their same gender parent.  Look at Chibiusa for starters or even the kids of Naoko's Parallel Sailor Moon Story.   Now I know neither of those things were written at the time, but there are tons of other examples across other media that fit the pattern.  Thus when the woman sees Endymion he looks very similar to his young father and thus the hint of recognition, while still knowing he is different enough to not actually be the same person she was in love with.  (Again as a monster after seemingly recognizing him she turns away from him and seemingly goes to find the actual person she loved).

But wait Endymion wasn't just anyone, he was the Prince of Elysion aka the Golden Kingdom  (Just known as the Earth Kingdom at the time).  So how could his parents have died like in this story?
- His father was the King/Queen of Elysion's brother, thus he was a Prince taken in by the King.
- His father was the King (or next in line to be), when he died Endymion's uncle became the new King and raised him until he was potentially old enough to rule.
- His father was unrelated to the King, but baby/young Endymion was taken in by him for some reason (Queen unable to conceive an legit heir?  Old friends?  Saved his life?)
Still works...plus it's also kind of fascinating that this could mean that not only is he destined to fall in love with Serenity...but he is also destined to be orphaned at a young age.  Poor Endymion...

With that said the person in the story could indeed have been someone else Endymion was related to such as an grandfather, or an uncle or something.  It's just the further you get away genetically the potentially less likely there is going to be that strong enough of a resemblance between the two which was necessary to make this jealous woman do her double take at seeing him...twice.

Seriously though, am I missing something or is there another explanation for how the Legend of the Lovers can actually have happened and make more sense?  I mean for the most part I certainly don't believe that it was the anime's intention to deviate so spectacularly from the manga and provide a potential explanation for Endymion having been born with powers and an connection to the Silver Millennium... (Keep in mind this was years before the Golden Crystal reveal of the manga, or the idea that he was essentially Sailor Earth).  Yet on the other hand I really have no idea what they were thinking.  If it was meant to be strangers then why have Endymion get recognized?   If it was meant to be Serenity/Endymion...there has been zero subtlety about anything involving them to this point.  They look EXACTLY the same in any dreams, visions, paintings, etc that have come up...so then why do the couple look noticeably different here if it was actually supposed to be them?  So I am at a complete loss as to what the actual intention was with this full story if the person responsible was not trying to sneak past their own Endymion origin story once it seemed like Naoko was not going to explain it...

your theory is confusing

I agree with Swan Cat, this is pretty confusing. I think you might be overthinking this a little bit. Interesting thought though!