In the State of Montana, the Title and Registration Bureau regulates the titling and registration of motor vehicles, including ATVs. Your ATV can not be titled without being registered and the State of Montana prefers to do both at the same time.
If you have recently moved to Montana or you have only recently acquired an ATV, you have 60 days from your beginning of residency or your receipt of the ATV to register it with the State of Montana. At the time of this writing, titling of your ATV cost $10 and registration was included in that fee if you titled and registered your ATV at the same time. If not, you will have to pay $10 for each procedure.
This is true whether you ride your ATV on publicly managed lands or private lands. The State of Montana keeps a listing of all ATVs reported stolen and at the time of registration the number on your ATV will be checked against the listing of stolen ATVs in the State.
In order to meet the requirements to ride your ATV on publicly managed lands, unlike many other states, Montana does not require that your have headlights or a muffler installed on your ATV. However, a spark arrestor is a required piece of equipment in order to ride your ATV. Riding on paved roads is illegal unless you are using the road to get cross to get on another ATV trail, additionally there is no State mandated minimum age for riding an ATV in the State of Montana.
Be safe and know the rules for riding in your State and instead of a fine, you'll have a great time out riding your ATV.