A lot of ponies suffered under Discord’s rule, possibly even dying until he was neutralized by Luna and Celestia.
Godzilla Dawn (or G-Dawn for short) sparked the first ever appearance of Godzilla in 1954. Thousands of people in Tokyo died and millions more were wounded before the monster was seemingly killed by the Oxygen Destroyer… only to show up a few months later.
Sombra’s Rise to Power and The Kaiju Outbreak:
Sombra overthrew the Crystal Empire’s original regime and enslaved its ponies. He was overthrown himself but his subjugation led to the disappearance of the Crystal Empire for a millenium.
The Kaiju Outbreak of the late 60’s into the early 2000’s led to a massive increase of the Kaiju population, thus bringing forth the establishment of G-Force.
Rise of Nightmare Moon and the Human-Kaiju War:
Luna’s transformation into Nightmare Moon led to the Elements of Harmony becoming dormant until Twilight and her friends reactivated them, therefore leading into the present events.
The Human-Kaiju War is still going on with no clear victor in sight and will probably continue to go on until either the humans win (very unlikely), both fight until they are driven to extinction (not as unlikely but still doubtful) or the Kaiju have won (huge possibility).