Pinkie Pie is a possible yandere to her friends from the series: My Little Pony: Friendship is Magic and Gravity Falls.
In the episode "Party of One" she shows a possible yandere side. Her usually hyper and excitable personality changes when she suspects that her friends no longer enjoy her parties. She begins to stalk her friends through town, eventually chasing her friend Rainbow Dash up a mountain. When she bribes the dragon Spike into telling her that her friends do not enjoy her parties anymore, she shows a whole different side to her.
Her hair, while usually frizzy, deflates and her coat and mane looses its bright shine. She begins to hallucinate and talk to inanimate objects, such as "Mr. Turnip" (A bucket of turnips placed on a chair), and she is violent, aggressive and rude to her friends until discovering the truth.
Her different personality and appearance from "Party of One" lead fans to call her "Pinkamena", which is her real name that she dropped after discovering the ability to feel joy as a filly. In "The One Where Only Pinkie Knows", she has strange hallucinations of her friends faces on balloons, glowering at her.
Often, she suffers from insecurities about her friends or special talent (Making others feel joy) that cause her to have "Pinkamena" moments. The episode, "Party of One", inspired an infamous MLP grimdark story called "Cupcakes". In the story, she uses a random generator to see who she will kill, then bake into treats and sell them in hopes to make others happy. Despite the story being noncannon, it still has official MLP origins.
In season 8 E03 'The Maud Couple' 17:11 pinky has a small breakdown and makes what might be a reference to Cupcakes.