The information that Opaline was possibly a pegasus - I say “possibly” - dates back to the beginning when her name and background were more or less unknown and background material was scrutinized to find any hint of her. That’s when an unfinished pegasus mare was discovered with a cutie mark reminiscent of Opaline’s. At this point, the cutie mark wasn’t fully formed, as seen by the discovery of it in the ANG movie itself. This led to a brief attention span that was quickly lost once the IDW showed Opaline as a unicorn when she pushed Fluttershy and launched what was the first attack on Equestria.
But I hadn’t forgotten this. The cutie mark itself was also unusual, although alicorns have wings, Opaline’s suggests a direct connection to flight rather than magic. We had learned from TYT 2.0 when Zinnea’s magic attacked Maretime Bay, that pegasi could have glowing wings when they channeled their magic for wind/air manipulation or physical attack.
Now on to the hypothesis. It arose after the discovery of Sunny’s mother as a likely born alicorn, categorized as a “sparky alicorn”, and that Sunny is actually also a born alicorn like Flurry Heart. This is reinforced by observations from the TYT episode where Nova was first seen, when Sunny for some reason managed to climb a tree without her parents being concerned about this. Sunny was still an earth pony until she “ascended”, which we now know was actually the removal of a seal on her. A recently unreleased footage from the movie work in 2018-21 showed that she could use magic like Opaline under special circumstances. That was when the other ponies could not use their magic, the alicorns being the exception.
What if it were possible for alicorns, even born alicorns, to be “deevolved” through special magical spells that could seal their alicorn powers so they could have a base mode - earth pony, pegasus and unicorn? An alicorn is a fusion of three pony modes - and we know that they are more of a magical creation than a physical one, they are much more deeply intimate with magic than others. Sometimes more like magical “constructs” than living chimera beings. This makes them incredibly powerful with a potential for godlike powers and immortality, they simply lifted themselves away from the rest of the pony kind.
The alicorns still cannot break free from their roots, and it is possible that Berrow and her co-authors had wanted to elaborate on this during their work. Anyone who followed G5 will notice that they had given the magical background a depth and explanation that was absent in G4, an increased focus on the magic behind the cutie marks - which of course was partly overshadowed by the rewrites that happened contrary to what the writers had wanted.
Nova was already an alicorn because she was the last castle manager in Ponyville. When she met and married Argyle, she was an earth pony when we saw her with her daughter and husband. We don’t know how Sunny could “lose” her alicorn powers that were “broken free” by the Unity Crystals that reacted to her actions - and the possibility that Twilight had sensed her nature through them. It is possible that these were sealed by Nova - which Twilight could break. What if Twilight was able to use such a spell on Opaline at the end of their fight after she had neutralized the fire alicorn?
Spike mentioned “a plan” that he never elaborated on, devised by him and Twilight on how to deal with the problem of Opaline, when they couldn’t find her after she had managed to escape. It involved isolating her from all the magic she could assimilate with the help of her talent as the element of Fire, why such a strategy that was meant to starve her out? Twilight went to great lengths to keep Opaline away from all magic, even “locking” her with a magical bubble as seen on Spike’s wall paintings. The dragons had to be brought to safety, because as long as Opaline had her cutie mark she could break the seal, as seen when she assimilated fire magic from Sparky.
As if Opaline had become a normal mortal pony instead of an alicorn, who would have died of old age while Twilight was busy ruling Equestria - and the rest of the world. But it seems that the Together Tree in the Dark Castle was very important to Opaline, as seen in the flashback when she tried to “activate” the tree with Misty who was still a young foal. Perhaps this tree allowed Opaline to retain her alicorn mode and thus remain immortal despite having lost almost all active magic.
If that was the plan, it’s easy to see why this was “erased”.
Berrow may have gotten the idea from a fimfiction many, many years ago when Flurry Heart had become a tyrant after a war between the crystal ponies and changelings. She was so feared, when she offered the position of minister to a power-hungry mare - choosing this mare to break her legs rather than accept the offer. This shook Flurry so much, that she went to Twilight.
Later in the fimfiction, when the last surviving changeling arrived in Crystal City, she was surprised to discover an aging Flurry on the verge of death. She had been stripped of her alicorn powers, including her alicorn mode, and had become a mortal pony. It is possible that Berrow and her fellow colleagues - she was just one of a large group who had been working on the future G5 from 2018-2021 - were inspired by this fan fiction.
It must have been an incredible battle between the two powerful alicorns, the element of Magic that controls magical knowledge and strength against the element of Fire that controls magical fire and temperature. Perhaps the battle was not in Canterlot, but elsewhere - and it may have turned out in Opaline’s favor, as she managed to escape in the end.
She only lost because Twilight, in desperation, brought out the Unity Crystals to strip her of all magic. The battle must have been a terrible ordeal for Twilight as she was able to strip the magic and seal Opaline’s own magic. For Opaline managed to escape, even when she was greatly weakened, powerless and perhaps even stripped of her alicorn magic, possibly becoming a pegasus.
There are interesting concepts of Opaline as a pegasus, not a unicorn, from the background work on G5. We learned many years later, when it was discovered that Sunny is a born alicorn because her Nova is also an alicorn, that it is a possible hypothesis that an alicorn can be completely stripped of her alicorn powers. Opaline managed to survive for many centuries despite the absence of magic, but only because she remained an alicorn - which she may have regained with the help of the together tree in the dark castle.
A canceled recording from 2018-2021 work on the ANG movie showed that Sunny could channel magic during a time when active magic was “suspended” for all the ponies. Opaline may not have had this luxury, because Twilight has always been thorough in her work, even under the worst possible circumstances.
It is canon that Twilight and Opalina had their own showdown when they fought each other, which ended with one era ending - and a new one starting. The two alicorns must have been so strong, that they put almost the entire world in fear, including Spike who may have witnessed this in horror. Which explains his downcast expression when Sunny mentioned this battle.
But in hindsight, we know that Twilight had not only won, but also started taking precautions to protect the ponies and the world. When she disappeared many years later, abducted into the starry sky against her will, she had been deeply loved - so much so that the ponies wanted her back.
Do you have an image of the pegasus mare?
Yeah I’ve seen nothing indicating otherwise
As far as I know, Opaline was born an alicorn, like any other alicorn in Skyros
Link? Cuz that sure doesn’t sound like something I’ve ever heard about Opaline
Where’d you hear the Opaline being a pegasus thing?
Ehhh, she got sealed away from entering Equestria for hundreds of years, and was a shadow of her former strength
She literally didn’t XD