Being that it's a month to month means that...their lease "expires" at the end of the current month, and "renews" at the beginning of the next month. The renewal is automatic unless you give notice of non-renewal.
Just like you would an annual lease, you would give them notice what the terms of renewal are regarding pets.
You have to give them at least 30 days in your notice. If you want to be nice, you can give them 60 days.
If your rent is due on the 1st of October, you need to give them a 30 day notice no later than September 1st. Since we're in August, you can give them a notice for October anytime this month as well.