Due to the sheer effort it has to put in to move its hulking body, and the image of fish bellies for eyes is a disturbing image that makes me think of a dead or soulless creature, as eyes are supposedly the window to the soul. The quote his stall narrows to rage is also quite a frightening quote, as I would not like to be around such a colossal creature when it is enraged. To add to the aggression of the bull, Clarke describes the smell of a heifer's fear sweet (from the point of view of the bull). The fact the bull knows the smell of fear makes the reader even more afraid of this absolute monster of an animal that relishes its counterpart's fear.
But the poem ends with an empathetic view of the bull describing his aggressive behaviour as a defence mechanism for his abuse as a chained animal with his crazy eyes churn with their (the fellow bulls in his memory) vision. Shows the bull's reaction to his nostalgia. The word churn describes the eyes moving possibly in a circular motion and adds to the whole air of rabidity and violence about the Friesian bull, and the word churn is a very meaty and heavy word, reflecting the whole persona of the subject of the poem. This shows that bull is indeed very affected by his flashback. Clarke also uses their vision instead of his vision, indicating that the bull craves to be with his herd again and wishes for the freedom that he no longer has.
The imagery in the poem is so clear, so specific at some points (e.g summer hay smells) that one cannot help but be transported into the mind of the bull and so can empathize with it.