Harry Potter: "Who keeps a mousetrap in a handbag?"
Neville Longbottom: "My Gran. Ever since she reached into her bag in 1947 and found a fanged gerbil in there."
Harry Potter and Neville Longbottom[src]

A fanged gerbil is, presumably, a gerbil with fangs. One of these found was put into Augusta Longbottom's handbag as a joke by her husband in 1947, after which time she kept a mousetrap in her bag at all times.