The description of the ferret in "Sredni Vashtar" as a god is an example of personification. Personification entails awarding human traits and features to an inanimate object or to something which is not, in itself "a person".
The story "Sredni Vashtar" is quite tragic. Saki draws literary traits similar to those of Roald Dahl, as far as tapping on the depth of human emotion, and the human psyche, in a very profound way.
The sickly 10 year-old in the story, Conradin, is lonely, neglected by his caretaker cousin, and is very much unloved. He is alone in the world and, as a result, he finds companionship in a smuggled ferret that hides inside a hutch outside of his house. The ferret, which itself is described as a polecat-ferret, is also dubbed "the great ferret" in the story to denote the sense of deference that Conradin begins to develop for it; a deference that touches on the obscure shifting, from an amicable appreciation toward the animal, to the complete worship of it.
Every Thursday, in the dim and musty silence of the tool-shed, he worshipped with mystic and elaborate ceremonial before the wooden hutch where dwelt Sredni Vashtar, the great ferret.
We are not sure whether the child has any supernatural powers, or if he sees something that the reader cannot. It is also unclear whether the child is mentally ill, as well as physically. Those things are not stated in the story. What is clear is that the ferret fills many huge voids that exist within the boy: loneliness, fear, despair, lack of faith and hope, lack of joy, and the loss of his innocence altogether at the hands of oppressive and mean caregivers.
When the boy "prays" to the ferret, he is filling the void that is caused by having nothing else to pray to, or hope for. The ferret becomes this symbol of everything that seems to soothe the internal turmoils of this child. Therefore, giving all of these attributes to a simple animal, rare as it may be, is basically stating that the animal would have the power to exert that much influence over a person. This is why it serves as an example of personification.