Mont. Code Ann. § 7-23-101

It is unlawful, when part 21, 7-23-4103, and this part apply, for any person to own, harbor, or keep any dog over the age of 5 months or to permit a dog owned, harbored, or controlled by the person to run at large unless the dog has attached to its neck a substantial collar on which is fastened a license tag issued by the authority of a county or a municipal corporation for the purpose of identifying the dog and designating the owner. It is lawful to remove the collar and license tag when the dog is under the immediate control of its owner or the owner's agent.