You need to call a plumber and have them install a one-way flap valve in the toilet. They look like the picture below.

Here is some good advice from the Internet Center for Wildlife Damage Management - the Rodent Proof Construction Manual written by three Agricultural Professors:

http://icwdm.org/handbook/rodents/RodentExclusion.asp

Drains and Pipes. Both rats and mice use drainage pipes or sewage systems as routes to enter buildings. Equip floor drains with metal grates held firmly in place. Grate openings should not exceed 1/4 inch (0.6 cm). Maintain 1/2-inch (1.3-cm) hardware cloth over sewer roof vents in rat-infested areas. If the sewer system is known to be rat-infested, a “Rat Guard” one way flap valve may be placed in toilets (Fig. 18). Sewer laterals should be checked for openings that could allow rodent entry. Smoke producing leak detectors are often used by agencies checking sewer lines for leaks or openings. If openings are detected, replace the pipe or wrap the pipe break with 1/4-inch (0.6-cm) hardware cloth and use concrete patching material to seal the area. Rain gutter downspouts are often used by rats to gain access to roofs. It may be possible to screen over openings at the base of downspouts with 1/2-inch (0.6-cm) hardware cloth or a grate, but this will require continued maintenance to remove accumulated debris, particularly where leaves and small sticks are washed from roofs into the gutter system. Flap valves have been used here too—swinging shut except when water is flowing. Openings to floor or driveway drains should have covers. Gutter and other drain covers must be kept clean of debris to prevent water backup.

(end quoted material)

Yeah, you need to call a plumber right away and have them take care of this. Good luck.

Kill it! KILL IT WITH FIRE!

Jesus, Mary and Joseph.

If you need pest control services, I have a recommendation for you. Aria Environmental- www.ariapest.com They evicted some rats for us recently and do a great job.

As for the drain issue, I guess you should talk to a plumber.

That is the stuff of nightmares.

OH MY GOD

Take photos.
It's hard to argue with photographic evidence.