So, you may of heard of theories of certain kid shows, right? Well, there was never one for the 3rd generation series of My Little Pony. Here's my attempt at this:
Rainbow Dash was always a happy pony, who was born with Argyria. She later went on to be the next Iris.
Minty was a socially awkward pony, who actually suffered from stendhal syndrome and bipolar disorder. She eventually fell into a major depression and took her own life, after accidentally breaking The 'Here Comes Christmas Candy Cane' one Christmas.
Pinkie Pie was imaginative, friendly and a fun-loving pony, sometimes taking the lead in some situation. She later became an addiction to alcohol and drugs, eventually dying from cancer a year later.
As for the remaining ponies? They went to other parts of life of course.
In total, the ponies were souls of people who died because of certain conditions. And as a result, everyone in Ponyville are citizens who lived their lives to their fullest and Ponyville is now their heaven. They may have not got chances to live in the real world, but lived good lives in the dream world.
In the dream world, that is...