The humans have crushed the evil but incompetent pony invaders. Equestria now belongs to them as their new state. Celestia and the other so called Princesses are all dead or put in their place. It is truly a glorious time for humanity!
Except... Twilight Sparkles know that this can't be right!

It's not just ponies the potion offers transformations of after all.

https://tvtropes.org/pmwiki/article_history.php?article=Fanfic.TheConversionBureauNotJustPonies