"The Horses" is filled with striking metaphors that bring its imagery to life.
For example, in line 4, the speaker says that the world is "cast in frost." This just means that the world is icy cold, covered in a thin layer of frost; the phrase "cast in frost" makes it sound like the world is a sculpture made from ice, however, emphasizing just how cold and still the landscape is.
The metaphors in line 5 similarly convey the harshness of this environment. The thick, gray, twisting fog of the speaker's breath becomes "tortuous statues" in the metallic, pre-dawn light. Later, the speaker says that "frost showed its fires": metaphorical imagery conveying the way the sunlight flashes brilliantly off the frost-covered land.
Many of the metaphors in the poem focus on the horses themselves, which the speaker compares to megaliths (large stones used in ancient religious practices), "[g]rey silent fragments," and the horizon. Their flowing manes are so still they might as well be "stone." All of these imaginative comparisons emphasize the unmoving, quiet, patient nature of the horses, who take no notice of the speaker's movements, nor the curlew's call, nor even the rising sun. Ultimately, the animals' constancy comes to symbolize the unchanging reliability of nature itself, which comforts the speaker in the face of the chaotic "din" of crowded city streets.
The speaker also uses a striking metaphor to describe the curlew's sudden, piercing call. Instead of describing this as a sound, song, whistle, or simply call, the speaker uses the word "tear." The word (which is also a pun) implies both the mourning sound of the bird's two-note call and also its violent edge. It's almost as if the sudden sound rips the silence apart, thereby prompting the sudden change in the sky as the sun begins to rise in earnest.
Likewise, when the speaker observes that "detail leafed from the darkness," there's a metaphorical comparison being made between the details of nature, which the rising sun makes gradually visible, and the organic growth of plants' leaves. The metaphor enriches the speaker's sense that the rising sun makes the natural world come alive in a particularly vivid, moving way.