Animals in horror are used in all sorts of ways. Scary animals can be the main villain or antagonist, such as in Cujo or Boar. Animals can act as more of a Harbinger such as in Pet Sematary. Or animal and monster subgenres may become intertwined so that we have giant animals such as King Kong.
Ultimately, animals being something that is to be feared is a natural direction to go with horror. And it brings a lot of complexity and cross-connections with other subgenres.