Voldemort’s mother, Merope Gaunt, could have created an obscurus by the time she was pregnant with Tom Marvolo Riddle/Voldemort. If you take away the fact that a young child typically creates an obscurus (although there is evidence Merope was surpressing her abilities when she was very young) then I think this theory has a lot of potential based solely off of evidence given to us through Harry Potter and the Half-Blood Prince and Fantastic Beasts.
History of the Gaunt family and Voldemort’s immediate relatives (mother, uncle, and grandfather on mother’s side): !!!(Spoilers below; Half-blood Prince) The Gaunt family as of ~1920’s were the last direct descendents of Salazar Slytherin consisting of three members: Merope, we already know as Voldemort’s mother, Morfin (Merope’s brother), and Marvolo (Merope’s father). We journey into the Gaunt household via the memory of a Ministry worker at the time named Bob Ogden. This memory is given to Dumbledore in the future, then showed to Harry (Half Blood Prince).
From the memory, we see that the Gaunt family is anything but cooperative and happy. Generations of inbreeding left the Gaunts violent, mentally unstable, and poverty-stricken. It is obvious that the father, Marvolo, is extremely abusive, mentally and physically, to his children- Merope especially. She showed lack of magical talent in her early years, and as a result was abused by her father, who called her a “disgusting little Squib”, among other things. It is presumed that she never went to a school to correctly learn how to use her magical abilities (although she has a wand) and that these abilities were continually suppressed due to her father’s abuse and belittlement.
By extension, it can be inferred that Merope had magical powers, but had never really used them nor developed them properly (until her father and brother were taken to Azkaban). Fast forward to after her family’s arrest and her subsequent freedom from emotional/physical torture (Merope is around 18 years old): her sights are set on a Muggle from the nearby village named, Tom Riddle (Voldemort’s soon-to-be father). She shows herself capable of significant magic when she enslaves Tom Riddle either with the Imperius Curse or a Love Potion (Potion is the most likely) for an extended period of time. This causes Tom Riddle to become infatuated with Merope and ultimately leads to their marriage and the creation of Tom Marvolo Riddle/Voldemort.
Once pregnant, Merope decides to free Tom Riddle from the “love” enslavement presumably because she can no longer handle the guilt of magically trapping him. Hoping that he actually fell in love with her after all the time they spent together or at least that he would stay with her for the sake of their unborn child, she lifts the charm. However, Riddle seemingly awoke into a nightmare and fled back to his real family’s home, abandoning the pregnant Merope. After her husband’s sudden abandonment, Merope soon fell into a deep depression and sold her valuables to survive in order to gain some gold (one of which was the Slytherin locket, which eventually becomes one of Voldemort’s first Horcruxes).
With her heart broken and her dreams in tatters, Merope stopped using magic altogether after Riddle’s desertion, not even willing to raise her wand to save her own life (this last bit becomes a driving factor for Voldemort in the future-being ashamed that his mother was a witch and couldn’t even keep herself alive after giving birth). Which finally brings me to my point: For her whole life, besides about a year, Merope has been supressing her potentially powerful magical powers. Near the end of her life, while pregnant, she supresses them even more due to the fact that she was so ashamed and depressed over who she was and what she had done with her magical abilities (presumably).
This could have meant that she was potentially harboring an obscurus her whole life, briefly expressing her magical abilities for about a year, then supressing them again causing her to ultimately turn into an obscurial soon before she gave birth at 19 years old. This could have caused her to die (along with her weakened will to live) and (maybe?) release the dark energy into Voldemort…who we know had violent/vengeful tendencies in his childhood (these characteristics are also found in Slytherin/pure bloods/inbreds).
We know that when an obscurial dies, the obscurus typically dies (and vanishes along with them), but what if it attached itself to the closest living thing it could find (newborn Voldemort) just as a piece of Voldemort’s soul did to Harry? Could it be that all of these factors in Merope and Voldemort’s past that ultimately caused Tom Marvolo Riddle to be so evil, the most evil wizard ever to live?
And…that leaves these more questions now: What happens to a child who is born with such dark energy? How does the obscurus manifest itself? Is it possible that she was harboring one for the majority of her childhood then re-manifested it by the time she gave birth? Is it possible that this cataclysmic event (releasing of obscurus) could have happened and no one in the orphanage saw? Or that someone modified their memories? Or they just acted as Muggles do, and tossed it aside as some strange, freak accident? It is unknown what happened to Merope’s body after her death…