Ditto is a Failed Mew Clone
There is this one theory which state that Ditto was a failed attempt by scientists to try to make a clone of the Mythical Pokemon Mew. We know that they were trying to make Mew and ended up with Mewtwo It is stated in the theory that Ditto was made right before the infamous Mewtwo.
The fans support this theory with the facts that they are very similar in colour and they both have similar shiny forms as well, light blue, and they both weigh 8.8 lbs. Ditto looks just a blob that could be a failed experiment, but it just couldn't take the actual form of Mew, therefore it stays as its usual blobby self. More support gathers from the piece of evidence that both Mew and Ditto are the only Pokemon that learn the move Transform.
I would love to hear your thoughts related to this theory, so do mention what you think in the comments. Until next time!