He didn't come up the pipe unless it is busted somewhere...but then you'd have bigger problems..
He more than likely came through a hole in the floor somewhere and climbed into the toilet...perhaps with mama possum's help as they usually cling to their mother's furry back to hitch rides..
We live in a concrete slab foundation home however we noticed the dirt on the side of the house had a tunnel dug out near where our master bathroom is located. Then we started hearing like these scratching noises under the bathtub and I actually saw a large possum in the back yard one night. Now last night while I was about to go to bed I heard our toilet seat bumping up and down in the guest bathroom. Upon further inspection it was a baby possum. How the heck it swam into the toilet I just don't know. Any recommendations on how to get rid of this pest. We want to fill the hole but don't know when the possum is in there or when it is out. Also how did a baby swim up the pipes into the toilet??? Confused and have no idea what to do. Please help.