So hear me out: Kou the Cow is the soul of the player's mother.

After visiting Bondee's one day, the mother was killed by the animatronics somewhere no one could see. The animatronics stuffed her into the Kou animatronic, which she possessed. The mother was deemed a missing person, and after days upon weeks of searching, no one had a single lead.

The mother's child went to Bondee's for his birthday, but not just as coincidence; he had a feeling she was there, and was going to search until he found her.

The child searched all afternoon, looking for his mother, but found nothing. The child did not want to go home, and wanted to continue looking. And so, the child hid inside the Kou animatronic, thinking no one would find him in there.

As day turned to night, the child waited until the coast was clear to leave; but what he didn't realize was that Kou was stationed right under a security camera. He was stuck. On top of that, the animatronics were now in attack mode, and would kill any intruders who showed themselves. The child was trapped. All of a sudden, Kou sprang to life. The mother was now in control, and, knowing full well that her child was inside, swore to protect him.

The sound of Kou jostling to life made the other animatronics head towards that area. All through the night, Kou kept her child safe by shutting the doors, dodging the security cameras and opening her mouth to let the child breathe. Once the clock chimed to 6 a.m. (opening time), the mother's soul left Kou, returning the cow to its lifeless state. Once this happened, the animatronic's jaw closed together, suffocating the child inside.

(I guess this is more of a fan-explanation of Bondee's than a theory, but still!)