Pfft was definelty not a dog.... You had to talk about the olfactory sensory imagery (olfactory = smell) that reminded the man of his past. If you noticed, when talking about the present there was a lack of smell, and at the end he "wiped his nose clean", symbolising his acceptance of the past and moving foward into the here and now. Main point of the poem: reflects the importance of nostalgia in overcoming the uncertainties of the present and ambitions for the future.
My interpretation anyways... If you said it was a dog, you can not be market wrong as it is your personal interpretation, yet it woudl be EXTREMELY hard to link with the question and build an argument.