Many years ago, after an S.C.A. event, we discovered a live guinea pig in his cage, among the left items. I wound up taking the wretched rodent home with me. Which is how I found out I'm allergic to guinea pigs. I rode home with 3 good looking men, red faced and sneezing.
The guinea pig spent the night in our basement, on a stool, where the cats couldn't get him. Ocelittle looked all over. She was certain the ultimate mouse was somewhere! If she could only find him. It did take a while to find the beasty's owner, but I wasn't sorry to see the last of him.
So, don't leave a guinea pig. Might I reccomend you try the Archie McFee company. They offer all manner of weird stuff for you to leave at the pub. Or perhaps you could check out Giant Microbes. They sell giant plush germs. Mononucleosis is really cute.