In their travels, Todd and Viola come across a sea of magically peaceful cows. We don't get an actual species name, but they're described as "twelve feet tall, covered in a shaggy, silver fur with a thick, fluffed tail at one end and a pair of curved white horns at the other reaching right outta their brows and long necks" (22.14). Okay, cool.
What's more important than their awesome silver fur, though, is the sound they make: They sing the word here in unison. Because of this, these cows represent the fact that peace and harmony do exist in the world, which is something that Todd and Viola need to be reminded of, since they're running from town to town from an army that threatens to destroy this very same world. We're told:
It's like the song of a family where everything's always all right, it's a song of belonging that makes you belong just by hearing it. […] If you have a heart, it breaks, if you have a heart that's broken, it fixes. (22.24)
It's a song of hope—and a symbol of hope, too—for two kids who could really use something bright to hold onto. Thanks, silver-furred cows.