At the beginning of chapter 2, the oldest of the colts "raise(s) his head" and "prick(s) his ears" because he hears "what sound(s) like the cry of dogs." The colt and the other horses with him, somewhat alarmed, move to a different part of the field where they can have a better view of the hunt. The narrator's mother then suggests that the dogs they heard may be involved in a hunt and may have captured a hare.
The horses then see a pack of dogs "tearing down the field" next to the one that the horses are in. The dogs are followed by "a number of men on horseback." The horses watch the hunt for a little longer, and they see a hare "wild with fright" run past them and towards the woods. Eventually the dogs capture the hare and kill it. The narrator then also notices that there are two horses "down," one "struggling in the stream" and the other"groaning on the grass." The one on the ground has a broken neck. The implication is that both of these horses have been scared by the hunt and have panicked and got themselves into trouble.
When the oldest of the colts raises his head and pricks his ears back at the beginning of the chapter, it is perhaps because he knows from experience the dangers posed by the hunt. He knows that it is sensible to be alarmed and to move out of the way of the hunt as soon as possible. As the narrator's mother says later in the chapter, the hunt "often spoil(s) good horses, and tear(s) up the fields."