Ferrets are not related to mongooses. Ferrets are Mustelidae , which is the weasel family . Mongoose are closer related to hyenas and cats. And according to their common ancestors, mongoose are even more closely related to dogs than ferrets and other weasels.
While they do have similar body types, ferrets/weasels and mongoose are not related in any way other than being carnivores.
Yep yep yep. The closest a mongoose is to a ferret, is 'Carnivora'. . . That's a BIG difference!