At the beginning of chapter 5, Percy has a dream in which barnyard animals either want to kill him or want food.
The dream is Percy's way of processing what happened in the previous chapter. There, Percy had encountered the Minotaur, which he calls the "bull-man" because his mother had warned him that saying the name "Minotaur" would give the creature power.
Percy's impression of the Minotaur is that the creature is highly animalistic. It smells strongly to him of the barnyard and of rotten meat. It charges him like a bull, true to its name, and has a powerful, furry upper body. Like many animals, it operates more through sense of smell than sight. It also lacks the intelligence and dexterity of a human. Percy can evade it, for example, by running sideways, as the creature is limited and can only run forward and in a straight line. Percy has what amounts to a bullfight with it, eventually winning by stabbing it with a horn. However, before this happens, the minotaur grabs his mother, and she disappears in a flash of light. The animals wanting food in his dream refer to Grover wanting to be fed during the fight.
Percy has just had a traumatic experience, and the dream he has relives it.