Here's my belated contribution to the discussion...the quotes at the beginning of Maus I and Maus II. What do they suggest as to why Spiegelman chose to represent the different groups of people as animals?
"The Jews are undoubtedly a race, but they are not human"
"Mickey Mouse is the most miserable ideal ever revealed... Healthy emotions tell every independent young man and every honorable youth that the dirty and filth-covered vermin, the greatest bacteria carrier in the animal kingdom, cannot be the ideal type of animal... Away with the Jewish brutalization of the people! Down with Mickey Mouse! Wear the Swastika cross!"
- German newspaper article, mid-1930'sG...out