Jack-O-Chica is the halloween version of Nightmare Chica found exclusively in Five Nights at Freddy's 4: Halloween Edition.
Jack-O-Chica is essentially just a reskin of Nightmare Chica. The changes made in it involve Jack-O-Chica being orange, rather than yellow, having an orange candelight glow out of its body, as well as having the Nightmare Cupcake replaced with the less-menacing Pumpkin.
Five Nights at Freddy's 4: Halloween Edition
She acts exactly the same as Nightmare Chica, with only a cosmetic change to her.
Jack-O-Chica's role parallels that of Jack-O-Bonnie, coming from the East Hall of the house. Unlike Bonnie, however, when the child's back is turned, she sends another animatronic, the Pumpkin, to do her dirty work. She is the slower out of her and Bonnie, and tends to wander into the kitchen, banging around dishes for unknown reasons. The child has to hold the right door shut to prevent her from getting in. If the child flashes the light at her when she is standing right outside the door, she will attack him, and if he turns his back on the door to look at the bed, Chica will have the Pumpkin enter the room and attack. The only way to tell she is at the door is to listen for her breathing. Holding the door will cause her to walk away, being unable to enter.