Okay, first semi-public draft of this lore ig

QUICK GLOSSARY:

New World - Setting of the game Monster Hunter World, largely untamed land, with varied locales
The Guild - Group of Hunters, Researchers, and support crew that are exploring the New World
Paolumu - A giant fluffy white bat-platypus dragon that can puff up their necks to float around and intimidate potential predators.
Aberration - Guild name for a fusion of a monster and a human
Abomination - Guild name for a fusion of two monsters
Eldertaint - Blue energy that has extreme, untapped power. A large burst of it was released when a group of hunters beat a giant monster (the final story boss of the base game of MHW)

CANON UNIVERSE (Monster Hunter World AU)

Former Human, and a Monster researcher, specializing in Paolumu
Deployed on a research expedition, and was studying a pair of Paolumu mates.
Both got attacked by a Legiana (monster higher up the food chain) and were both gravely injured
Maya approached them, and tried to use her research to perform palliative care.
As she lay between them, a flash of blue light shot flashed in the distance, and she blacked out.
When she came to, she was fused to the two Paolumu, which had themselves fused.

Turns out that it wasn't an isolated case, and people & monsters had that happen all over the New World.
After hiding in fear for about a week and discovering she had an amount of control over the shape of her new form, the Guild found her, and with the help of another Aberration (Maya's research partner Elara, who was recovered on the day of the event) was able to recover her.
The Guild takes special interest in the fact that she is both an Abomination and an Aberration, and her ability to control her form.