It all started with Team Rocket. Although you couldn't say that it was Team Rocket's fault.
It was way back when they were trying to create Mewtwo, the most powerful Pokémon of them all. They were running some experiments with some Mew cells that they cloned. They were trying to find a shortcut. They were trying to find a way to create Mewtwo with the limited cells that they had.
They charged the Mew cells with a high concentration of radiation. Not the safest thing to do. It caused the cells to multiply at an astonishing rate. It created a big blob. The first Ditto. But this wasn't like the Dittos we know of today. This one was different. It was blue, and it didn't have any facial features.
The Ditto transformed into the scientist that created it. It was a perfect imitation. The scientist was startled and started to back away. The Ditto got up off the table and grabbed the scientist by the throat. The scientist gasped for breath. The Ditto choked the scientist until he passed out. The Ditto then dropped him to the floor. Ditto kicked the unconscious scientist out of sight.
Ditto looked at what the scientist was working on. Ditto got to work on multiplying some more cell cultures. It used cells from its own body. Ditto created more versions of itself. They were pink, blobby creatures with beady eyes and big mischievous smiles.
"Go my children," Original Ditto commanded, "Go and make mischief and chaos!"
The Dittos hopped off the table and stormed the lab, transforming into the first Pokémon or humans that they saw. They were destroying the lab. It was at this time when Blaine finished creating Mewtwo with the cells from his arm.
All of the Ditto chaos caused the capsule encasing Mewtwo to become unlocked. Mewtwo then destroyed the lab. Blaine was one of the lucky survivors to escape.
After Mewtwo's escape, the Dittos quietly followed Mewtwo to Cerulean Cave. Wild Dittos can be found there to this day. Original Ditto is still on the move. Because Dittos can't breed with each other, Original Ditto is the reason that we can find new, wild Dittos. One of Original Ditto's greatest accomplishments is the training of The Ditto Five hiding in Konikoni City in the Alola region.
Original Ditto yearns to cause the downfall of all humans by infiltrating their governments and sowing seeds of chaos. If someone you love is acting strangely, or they're not talking to you, they've been replaced by a Ditto. That, or they're on drugs or something.
THE END