If a tenant has a pet which is not allowed by the rental agreement, the landlord may give the tenant a 3- day notice specifying that the tenant must either get rid of the pet or vacate the premises. For the tenant, temporarily removing or "hiding" the pet is not a good solution. Following the initial notice, if the landlord discovers any unauthorized pet in the rental within the next six months, they can give the tenant a five-day notice to move out, with no opportunity to remedy the situation. If any unauthorized pet causes damage, the landlord may give the tenant a 3-day notice of termination for destruction of property, in which case removing the pet would do no good and the tenant should find another place to live.

Source: 70-24-422

Landlords are allowed to limit any kind of animal that isn’t a service animal according to the Montana Code, which only lists dogs and miniature horses. Pet deposits fall under security deposits in that there is no limit to what landlords can charge.

Source: 49-4-203 and 70-25-101

You are responsible for the behavior of any of your guests. If that guest violates your lease agreement, it counts as a violation on your behalf even if they’re just visiting for a short time period. Your landlord will probably issue a written warning that you remove the pet in 3 days or face lease termination. Even if you remove the pet in fewer than 3 days, this first written warning will make any future written notices repeat violations.

Source: 70-24-422

A very common requirement in the lease is that every tenant is liable for rent, utilities and damages, so if one member of the lease gets a pet, you might be held liable if you don’t inform your landlord or your lease forbids it.

Source: 70-24-422