Symbolism, Imagery, Allegory
Pop quiz: what do Billy Pilgrim and horses have in common? They're both naysayers! (Hardee har har... ugh. Sorry.)
That's not just a horrific joke, it's also a lie. Billy Pilgrim and his four-hoofed friends have way more in common than just that.
After the bombing of Dresden, Billy Pilgrim and several POWs return to the slaughterhouse to pick up souvenirs. Billy doesn't actually spend much time looking for things; he simply sits in a green, coffin-shaped, horse-drawn wagon the POWs have been using and waits for his comrades. As Billy lies in his wagon in the afternoon sun, two German doctors approach him and scold him for the condition of his horses.
The animals are desperately thirsty, and in their travel across the ashy rubble of Dresden, their hooves have cracked and broken so that every step is agony. The horses are nearly mad with pain. Billy weeps for the first and last time during the war at the sight of these poor, abused animals (9.19-20).
Given that this scene is the only time Billy cries during the whole war, it must be pretty significant. In fact, the parallels between the horses' suffering and Billy's own seem striking. These horses have no way of understanding the destruction around them, nor the orders being given to them. With no way of protesting their treatment, they obediently keep walking through the ruins of Dresden even though every step on the sharp rocks damages their hooves. Like Billy himself, the animals are innocent victims of great suffering without ever understanding why.
No wonder Billy finds himself in tears.
There is also a parallel between the horses and Roland Weary, the first character we see die in the book. Weary is the bully who attempts to shoot Billy before the Germans capture both of them. When the Germans take Weary prisoner, they force him to exchange his excellent boots for a pair of wooden clogs a German recruit is wearing. The clogs are so rough on Weary's feet that he injures himself marching, gets gangrene, and dies. Weary's bloodied feet appear at the beginning of Billy's wartime experience, and the horses' cracked hooves at the end.
Perhaps the suffering of the horses reminds Billy of all of the terrible, pointless pain he has seen in this war, starting with foolish, violent Weary and ending with the Dresden firebombing.