The attack happened Thursday afternoon, when Noemi Stefenatti was out walking her dog.
Stefenatti said the attacking dog grabbed her pet — an eight-year-old border collie named Lucy — by the neck and attacked it.
"This poor animal couldn’t defend himself, because he’s not used to these attacks," Stefenatti told CBC News.
Stefenatti said her young granddaughter tried to intervene, but she pulled her away.
The attacking dog got away, but Stefenatti is hoping that police can catch the animal and charge its owner.
The dog was last seen in the Lawrence Avenue and Royal York Road area near Chapman Valley Park.
Stefenatti said the attacking dog was not with an owner, but was wearing a thick burgundy collar.