I had a ferret who liked to climb in through the port of my NHT VT2's and fall asleep. He never did any damage, but it took some heavy bass to coax him out of there! I would up buying some plastic plumbing grates at home depot, sanding them smooth on one side, and using double-stick tape to stick them over the ports. Believe it or not, when done they looked like the belonged on there, and I never heard any port noise caused by the grille.
Animal damage to system
After a hard day of work all I wanted to do was relax and watch a movie. I was 2 seconds into the movie and the protection circuit of the receiver activated. Come to find out that the cat had chewed all 3 of the front speaker cables, 1 phone line and 2 coax cables. What kind of damage have you experienced with animals and your system?