Ash's Pikachu is actually his father.styLIShT I have been thinking about this ridiculous conspiracy for a few months now, this is quite popular in the pokemon community. Keep in mind that this is obviously just for fun, there may be loopholes but hear me out on this one:
Ash’s father was a friend of Professor Oak’s in Pallet Town, much like Ash is now. While working on a risky experiment involving a Pikachu (as a power source or as the specimen), Ash’s father somehow got in the way of the experiment and his soul/essence was placed inside the Pikachu, leaving his own body either dead and empty or replaced by the Pikachu’s essence, which would make him seem crazed and/or deranged. Oak panicked and because of his close relationship with Delia (his student’s wife who is Ash's mother), he hid the body (in either state) and claimed that Ash’s father had gone off to “continue his adventure as a Trainer” and then hid away the Pikachu in shame or fear. However, as he knew Ash when he was a child himself as seen in the Celebi movie, Oak knows that Ash will eventually have a Pikachu and decides to give him the one from the failed experiment. Whether Oak knows the identity of Ash’s Pikachu as his father is irrelevant. Either way, he wound up giving the Pikachu to Ash because he knows that Ash will own a Pikachu in the future.
Once released from the Pokeball, the Pikachu attacks randomly and even makes fun of Ash. He may have temporary memory loss at this stage or he may just be very scared and not in control of his new body outside a Pokeball. Upon getting to know Ash and Delia better, he may remember his past and decides that he doesn’t need to be afraid, as his family still loves him. Here, we can argue that Delia may know the true identity of Ash’s Pikachu. If she (as a student and friend of Oak’s) also knew about the experiment, she may have her guesses as to why the Pikachu is so special and where her husband really went. If this is the case, she decided to let Ash start his Pokemon journey despite her motherly fears because she knows he is in good hands with his father.
Because he has a human’s soul and knowledge, Ash’s Pikachu is stronger and kinder because he can think abstractly about things and doesn't rely solely on his trainer. He also dislikes Pokeballs because he is a human unused to the Pokeball procedure and prefers the freedom to be with his son. Although he can never truly signify who he really is, Ash's father is still able to protect and teach his son throughout the young trainer's adventure.
In Latin America, Cartoon Network has a "Biografia Toon" (Toon Biography) segment, which consists of biographies and histories of cartoon characters. According to the segment, Pikachu is the result of an experiment conducted by Professor Oak and allegedly destroyed the lab where he was created in a momentary loss of control.
Delia knew Spencer Hale, another student of Professor Oak’s and she obviously cared for him as seen in the third Pokemon movie. From this information, we can construe that she may have known other students/trainers familiar with Professor Oak and perhaps fell in love with and married one of them.
Pikachu and Delia are fond of each other and Pikachu gives her more attention than Ash or his other Pokemon at many of their reunions. Also, we can see many instances of Pikachu fondling Delia. This fondness may be because Delia is aware of Pikachu’s true identity (or is guessing) or because Pikachu makes the extra effort to be nice to her.
According to Bulbapedia, Pikachu has a set language for many repeated words. He also seems to understand human speech and body language better than other Pokemon. Unlike most of Ash’s other Pokemon, Pikachu acts in a fatherly role to others, namely Togepi and Axew. He is also very mature and is usually the leader when Ash isn’t around. He often tries to “talk things out” with others before he fights, as seen in Pikachu’s Vacation while the others simply want to battle.
Pikachu has a particular dislike of Team Rocket’s Meowth. He deliberately wants to cause him and Jessie and James harm, as he is protecting Ash. Other characters don’t seem to have such a strong reaction to Team Rocket, not even Ash.
Ash and Pikachu have very similar personalities – eager to fight and test themselves against others, but just as eager to meet new friends and help each other. This may be because they share genetics.
Pikachu is incredibly loyal to Ash and even decides to keep traveling with him instead of staying with other Pikachu in Pikachu’s Goodbye, which is the 39th episode in the original series despite many of Ash’s other Pokemon deciding to stay behind or be freed in similar situations.
Pikachu does not want to evolve. Because of the radical physical changes that take place in Pokemon (as well as possible mental ones, as displayed by Ash’s Charmeleon), Ash’s father may be afraid to evolve as his essence may disappear forever.
Pikachu has an odd fondness for ketchup, a food otherwise reserved for humans. This may be due to Ash’s father remembering eating ketchup as a human. It would be a rare condiment on their camping adventures, so he revels in its flavor and memories whenever he can get it.
Pikachu is the first to cry (even before Ash's human friends) in the first Pokemon movie when Ash is turned to stone and "dies"
Summary: Ash’s father didn’t abandon his son, instead his essence was accidentally transferred into a Pikachu and he is now Ash’s closest companion.
SHOOK - TwilightKitsune