As a poet, Ted Hughes uses language in a very specific manner; strong descriptions and simple, unadorned prose allow deeper meanings to come from his texts. In "The Rain Horse," it is the power of his descriptions that creates most of the atmosphere:
The wood hummed and the rain was a cold weight, but he observed this rather than felt it. The water ran down inside his clothes and squelched in his shoes as he eased his way carefully over the bedded twigs and leaves. At every instant he expected to see the prick-eared black head looking down at him....
(Hughes, "The Rain Horse," Amazon.com, emphasis mine)
By showing the wood with dripping cold rain and close trees, "bedded twigs and leaves" and the ominous specter of the attacking horse hanging over him, Hughes creates a sensation of claustrophobia. A forest is normally a wide-open, immense space, but the man has nowhere to run because he feels the wood is alive, waiting for him to trip up or stop running. Hughes uses powerful adjectives and nouns to create a living space rather than a simple description. The wood here is not just a place for animals to live in, but a hostile force of its own, impeding the man's progress and wearing him out with brambles and the constant rain.
another majaor theme would be reality v/s realism.