Vonnegut makes use of imagery and simile to describe Billy Pilgrim's experience behind enemy lines:
Billy had never seen Weary’s face. He had tried to imagine it one time, had imagined a toad in a fishbowl. [...] They had heard the dog. They had heard men calling back and forth, too—calling like hunters who had a pretty good idea of where their quarry was.
Vonnegut describes in detail what is (not) seen and what is heard, specifically the German soldiers' hunting the Americans. Slaughterhouse-Five is ladened with imagery, especially imagery of World War II and its aftermath. Any number of passages could have been used to discuss imagery in the story, but dogs come up a few times, specifically in the sections detailing the war.
Part of what makes Slaughterhouse-Five so moving is its ability to place the reader in the shoes of Billy Pilgrim and the other soldiers, and Vonnegut's imagery helps make that possible. Comparing the men to hunters is barely a simile because that is very literally what they are doing: hunting the Americans. The sounds, sights, and smells of war dominate the sections of the novel about war, and this passage is no different. The novel's ability to succeed as an anti-war novel is predicated on its ability to make the reader feel the horrors of war, and figurative language like imagery helps Vonnegut do as much.
Even the dog described above becomes a part of Vonnegut's anti-war thesis outside of the instance of imagery: later the dog is revealed to be a pet owned by a local resident and borrowed by the soldiers. Thus, even the seemingly fearsome dog helping the Nazis root out Americans is merely a pawn. Nobody wins in wartime, dogs or people alike.