Coco Chanel is Mandi Weatherly's chihuahua. She has clearly named her after the famous French fashion designer.
Mandy regularly brings her into North Shore High School in a purse.
Mean Girls 2
In her first appearance in the movie, Abby Hanover fed Coco Chanel in the cafeteria, which caused Chanel to throw up in Mandi's purse. Enraged, Mandi throws the entire bag, with her dog inside of it, in the trash can. However, Quinn Shinn takes her and the purse out of the trash, adopting the dog.