Mont. Code Ann. § 81-4-215

If any cattle, horses, mules, asses, hogs, sheep, llamas, alpacas, bison, or other domestic animals break into any enclosure and the fence of the enclosure is legal, as provided in 81-4-101, the owner of the animals is liable for damages to the owner or occupant of the enclosure if the owner or person in control of the animals was negligent. This section may not be construed to require a legal fence in order to maintain an action for injury done by animals running at large contrary to law. In the case of trespassing animals in a herd district, the liability and damage provisions of 81-4-307 apply.