The Hamster in Butt World is an alternate dimension that appeared in the episode Rixty Minutes.
In this universe, all of the living creatures are hamsters and all of their homes are humans. The hamsters all live in people's butts and they spend basically all of their lives in there. It is seen in a photo that inside the humans is a little room, with TV, couches, and other household items, with stairs leading outside to the butthole.
The Smith Family was watching this on the inter-dimensional TV, and asking Rick Sanchez a bunch of questions about it. Rick eventually got fed up with all of them, because he hasn't ever been there either and took them all to visit the dimension. The family all had a fun vacation there and all of their questions were answered.
- The concept of hamsters living in people's butts could be a reference to the sexual activity, Gerbilling, where people will use a tube to insert a live gerbil, hamster, guinea pig, etc. into their rectum and have them walk around in there for the stimulation and pleasure.