The main theme that Ted Hughes uses for "The Rain Horse" is Man vs. Nature. The unnamed protagonist takes a shortcut through the wood and is beset by a black horse and by the constant rain and thick trees.
The success of this last manoeuvre was restoring his confidence, but he didn't want to venture out in the open field without making sure that the horse was just where he had left it. The perfect move would be to withdraw quickly and leave the horse standing out there in the rain.
(Hughes, "The Rain Horse," Amazon.com)
Because he sees purpose in the horse's attacks, the man gives himself a similar purpose; survive. His anger at the wood and rain is unreasonable because they cannot act with purpose, but the horse can, and it gives him a reason to run, to fight back, and to escape. Without the horse to push him, the man might have ignored the rain and slept in the wood; at one point, he almost slips into hypothermia from the cold, but the memory of the horse wakes him up and drives him to action. With this purpose, the Man fights Nature and, while he doesn't necessarily win, he reaches his destination at the end.