The Tralfamadorians are a race of weird aliens who abduct Billy and take him to their planet. Once there, he forms the main exhibit in the planet's zoo, where he's expected to mate with a young lady who's carved out a successful career in what can euphemistically be described as the adult entertainment industry.
In many ways, the Tralfamadorians are just like Billy's German captors in the POW camp. For one thing, they're prurient and don't allow their prize specimen any privacy. They also strip Billy of his freedom, preventing him from making any meaningful choices in life.
Physically, however, these aliens are—literally and figuratively—on a different planet from the Germans. As Billy describes them in his letter, they're about two feet tall, green, and shaped like plumber's friends, which are a kind of rubber suction-cup used to unblock sinks, drains, and toilets. The Tralfamadorians' suction cups are on the ground while their shafts point to the sky. At the top of the shaft is a little hand with a green eye in its palm. Though the Tralfamadorians may have abducted Billy, they're quite friendly creatures. They can see in four dimensions and have a lot to teach earthlings.