Just as the title suggests, the theme of this poem is horses. For the speaker in the poem, horses "seem to represent a strength of will and a natural grace that humans would do well to emulate."
When the poem begins, we can almost feel the chill as the speaker walks through the frosty woods. When he comes upon the horses, they appear as if statues in the cold dawn. When the sun breaks, however, the horses seem to take on a magical element. The sun is burning off the frost, and the steam is rising off the horses, giving them the appearance of movement and power without their having to move. Masterplots (linked below) sums it up this way:
While the narrator has described himself as empty and stumbling about as if he were “in the fever of a dream,” the horses appear calm, sure of their place in the world, able to endure all things. The poem ends with the narrator hoping, in a sentence construction reminiscent of prayer, that he will always remember the horses. Significantly, he now identifies them as “my memory.” They have become something both personal and abstract, and they seem to embody a spiritual resilience of which the narrator did not seem capable in the first lines of the poem.
The theme of the poem The Horses rests in a contrast between the conditions of light and dark, night and day and most importantly between stasis and movement and silence and sounds, that break it. Hughes's sur-real animal world is located in the in these junctional points of reality and fantasy. The horses in the pem arrive like a flash in his memory as he recalls an almost epiphanic vision of a genesis of sorts--the making of the world from the grey silence of chaos to a world of radiance, with the horses coming alive in the process. As always with Hughes, the shher power and energy of the animal-world (however violent) remains his central theme, but in this poem, much like the poems about the crow, Hughes sees a cosmic mystical force of creation in the horses who set the world on motion.