Adult Swim's Rick and Morty is one of the brilliant and mind-blowing show that I've watched. The sci-fi show which involve multi-dimension travel had lead the fans to many theories. Diving through these theories, I found some that sound convincing.
Evil Morty is Rick's original Morty
After having been missing for nearly 20 years, Rick Sanchez suddenly arrives at daughter Beth's doorstep to move in with her and her family.
From the summary of the show we know that Rick had been missing for 20 years. The fact that Morty is just 14 years-old makes us question this scene from the show:
How did Rick hold baby Morty if he's missing for 20 years?
“A cocky Morty can lead to some real problems. It can be a real bad thing for everybody.” - Rick
Does the "cocky Morty" means the evil (Rick's original) Morty?
Mr Poopybutthole
Although Mr Poopybutthole is described as "the long-time family friend of the Smiths" we only saw him in the episode Total Rickall. He was revealed not a parasite when shot by Beth when we all thought that Mr Poopybutthole is one of the them. So, how did he pop up from nowhere? A theory suggest that Mr Poopybutthole and THE WHOLE SMITH FAMILY in that episode are from another dimension! The theory takes a lot of explanation about the multi-dimension thingy and previous episode. I'll just leave a link here :)
Morty is actually as smart as Rick?
This theory is somehow correlated to the Evil Morty theory. Evil Morty has the ability to control Evil Rick by using whatever he had built. This has proven that Evil Morty has the intelligence to invent those gadgets just like Rick. However, it just proves that Evil Morty is smart, how about the rest Morty?
Remember the "Morty Waves"? Rick told Morty that their brainwave cancel out each other so that he won't be discovered by those who wants to hunt him down. In the first thought, we might think that the "cough Morty Waves" is just an implication that Morty is dumb. If we take a look on how waves work, we can know that Morty is not as dumb as Jerry but he is as smart as Rick!
A destructive interference occurs when two wave of the same amplitude but different phase meets one another. If brain wave(intelligence) in the show works like the actual wave, this theory might seem logical.