Rabbits don’t have a sac around their neck. The area you are likely thinking of is called the dewlap, which is a fold of skin found on the underside of the neck of some rabbit breeds.
It is not a sac, but rather a loose flap of skin that can sometimes be mistaken for one. The dewlap is more pronounced in some breeds, like the English Angora and the French Lop.
It is important to remember that rabbits have very delicate skin and should be handled with care. If you are concerned about your rabbit’s health, it is always best to consult with a veterinarian.