It is not clear from your post whether your roommates were diligent in reporting your pets to their landlord, who was slow about requiring a pet deposit, or if they were hoping that their landlord would not figure out that there were pets in the unit. You also haven't told us why this is suddenly a pressing issue -- is the actual issue that their landlord just discovered their deception and has demanded the money?
In Utah, a landlord can terminate a periodic tenancy on fifteen days notice prior to the end of the rental period (e.g., if rent is due on the first of the month, notice must come at least fifteen days before the end of the month). See Utah Code, sec. 78B-6-802(1)(b)(i). The Utah Courts website indicates that notice must be in writing. They cannot commence an eviction action unless they serve you a statutorily valid notice.

