The beloved hamsters of Butters Stotch, also affectionately known as his minons, are his two favorite pets. He has glued little caps and masks on them to simulate their status as his minions of doom. They have a tendency to scamper off when left unattended. They're also sometimes kept in Professor Chaos' secret base.
Butters' pet hamsters can be found both inside his house and inside Professor Chaos' secret base at the U-Stor-It facility.