Nature in "The Rain Horse" is simply uncaring; there is no thought to the rain or mud, the thick forest or the wild horse. Without reason, Nature cannot be said to be intentionally aggressive; instead, Nature is more of a bystander, doing nothing to help or hinder the man aside from simply existing.
In contrast, the man's character is entirely purpose and emotion; his initial decision to enter the woods as a shortcut comes from his reaction to the rain, and he finds himself personalizing Nature in his anger:
A wave of anger went over him: anger against himself for blundering into this mud-trap and anger against the land that made him feel so outcast, so old and stiff and stupid. He wanted nothing but to get away from it as quickly as possible.
(Hughes, "The Rain Horse," Amazon.com)
This anger allows him to feel personally attacked by both Nature and the titular horse, which seems to follow and attack him. In reality, the horse may only be defending its territory, and rain has no mind of its own; only his own ego allows the man to attribute purpose and reason to Nature and Animal where none is likely found.