A brief tangent on Japanese mythology: In the famous story of Momotaro the village where he is from is frequently raided by a marauding band of Oni from the Island of Oni. Though not specific to this story, in some versions their leader is the infamous Oni King Shuten Doji.

According to legend Shuten Doji was the child of a human woman and the many headed serpent Orochi. He was born with Teeth and Hair, was able to walk and talk at the level of a 5-6 year old, and had the strength of a 16 year old! He was a prodigy and yet he was still shunned, his mother told him he should be ashamed of his giftedness, and try to fit in with everybody else.
At age 6 his mother abandoned him and he lived in a monastery with Buddhist Monks, where he underwent monk training but would sneak off with other less devout monks and drink Sake which was forbidden, and despite being just a child he was able to drink grown men under the table, but soon he was hated by everybody in the monastery.
One day at a religious festival he got his hands on an Oni mask and decided to go around scaring people and making mischief while wearing the mask, but by daybreak he tried to take the mask off but it would not loosen or budge, it had become permanently fixed onto his face! He had acted so much like an Oni that he turned into one overnight and fled the village in fear no longer being able to be accepted by society, though he never was accepted by anybody anyway, now he just looked on the outside what everybody already thought of him.

Sometime in his many years of exile, Shuten Doji became the most powerful Oni in Japan who threatened the Emperor himself before eventually being slain by a Samurai.

According to recent studies of Japanese history and mythology, people are starting to see Oni in a new and more humanized light. Real Oni were not Demons and Ogres, but Outsiders: Foreigners, ethnic and religious minorities in Japan, and people who refused to submit to the imperial court or conform to traditional Imperial Japanese customs.

Electivire is partially based on the Oni, specifically the Thunder God in Shintoism Raijin and given it's shaggy Ape-like appearance it may also be based on a specific type of Oni called the Namahage who wear Masks and a straw cape. They are part of a New year's ritual where they would go door to door in groups of three asking if there are "any naughty children around" and scare kids into good behavior.

I imagine Captain Kinoko's backstory is basically the same as Shuten Doji, except instead of literally turning into an Oni he's just has the reputation of an Oni. He was Born a Prodigy, shamed for his giftedness, grew to resent conformity, and eventually decided if pokemon were going to hate him then why bother being anything but a monster.