(A fan-theory thing or whatever if it can be canon or something.)
The original Ao Oni was Raki, one of Mori's great-great-aunts. Raki was a fairly skilled doctor and often used her home as a 'hospital' of sorts where she would help patients, though it is known that over the long period of time that she was a doctor, many patients would die from their wounds or sicknesses (Which were caused by the Separ Monsters), and their spirits would become trapped in the house due to the Book of Shadows being in there. In her home, she was affected by the Blue Curse, brought about by misuse of a forbidden spell from the Book of Shadows, one that was supposed to make her very powerful. The effect of this curse essentially turned her into a demon, changing the style of her DNA completely. The effects were slow at first, as she begun to lose all of her hair, and her skin began to change its' shade, and her eyes began to slowly grow, and her teeth all fell out and were soon after replaced by large sharp ones that bulged out of her mouth, and her body began to grow in size and build (most notable her nose grew to be fairly large), a process that went on for about four months, until she became the creature known as the Ao Oni. The Blue Curse also affected her personality, making her violent, bloodthirsty, manipulative, and hateful.
However, her body quickly became unstable with the effect of the curse, and her body exploded, releasing her spirit all over and filling the area with further curse. Some time in between -53 and early -51 ADC, the 'Ao Oni X' came across her home. This Ao Oni X was the unfortunate product of the Wonder Jungle trying to improve on the known species of the planet by creating monsters of the like, as it had by creating humans in correlation with Gridmasks, the moment that the change of Raki's DNA had changed. The spirits of the mansion took over the body of this Ao Oni X, trying to provide it as a suitable host for Raki, but this failed, and killed it.
Later, Mori found the Book of Shadows, and took it with him as he left the home. This led to the release of many of the spirits from the mansion (excluding that of Raki and the Ao Oni), and a notable amount of energy resulting from it, which attracted the attention of Revelian, bringing him to the place. He found the body of the Ao Oni, which called to him through the spirit of Raki, and he then created the Ao Oni Original from it, and later the Ao Oni X. Using the DNA from the newly created Ao Onis, Revelian also later conducted experiments to create others of the like, though those that went without a form of 'base' to occupy turned out as failures. So, experiments were conducted to introduce the DNA into living beings, changing their genetic code and turning them into Ao Onis of types. Some of the experiments were done on creatures like bats and such, though a majority of them were performed on living beings.