A virus that transforms the infected into ponies swept over the world. Despite her best efforts to catch this disease and go pony, Ari was not among those transformed. With transformation magic banned forever and demonized by most of society, it appears she's stuck as a human permanently.

Only one group is in favor of turning more humans into ponies: the 'Shimmerists'.
...But they are very pro-ponifiation.

This story more or less takes place in the Pandemic universe. However, it's been updated to take place in the real 2023 and I haven't read the sequels. It's essentially a non-canon spinoff from the original.