In order to determine the role of the polecat-ferret, the Houdan hen, and the empty barnyard in Conradin's life, we should examine how these things come to be significant in Conradin's life. Upon close reading of the text, we find that Conradin lives with an emotionally distant and indifferent guardian, Mrs. De Ropp.
Conradin's guardian seems to enjoy tormenting her charge with 'coddling restrictions' and undue prohibitions against any of the typical childhood privileges children often enjoy. For example, he is not allowed to pluck any of the fruit from the trees in the yard; his life is one of tedium and a constant cycle of medicine-taking, due to a proclamation by the 'effete' doctor that he will not live out another five years. Any new joy he discovers is soon denied him 'for his own good.' As a result, Conradin is lonely and depressed.
He uses his rich imagination to weave a wonderful world of adventure, strength, and light to counteract his dismal situation. The empty tool-shed becomes the avenue for housing all manner of imaginary characters, both from history and his own creative powers. In this secret imaginary world, he is allowed to explore new experiences without being criticized for it. Conradin views the tool-shed as a haven from his meddling guardian.
On the Houdan hen, the boy 'lavished an affection that had scarcely another outlet.' The lack of emotional warmth in Conradin's life leads him to transfer all the expressions of human affection to this tame creature. The hen is Conradin's friend.
We are told that the polecat-ferret is Conradin's 'most treasured possession.' The worship of this 'secret and fearful joy' becomes Conradin's private obsession. Why? The narrator tells us that the polecat-ferret was a god who 'laid some special stress on the fierce impatient side of things, as opposed to the Woman's religion, which, as far as Conradin could observe, went to great lengths in the contrary direction.'
The polecat represents all Conradin's forbidden inclinations: the inclination to be strong, fierce, and fast. Conradin is not allowed to be any of those things because of his illness. His daily life is a constant curbing of all the necessary virtues that he admires (and that he himself would like to have, notwithstanding his illness). The polecat plays the role of the little boy's alter ego; his strength becomes Conradin's strength, his fierceness, Conradin's. This wild creature makes Conradin feel less helpless.