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There is a general problem. Django is the backend of your application. The default interactions of django happen when you reload or load the site. If I understand you correctly you want dropdown2 to display "Dogbreed1" and "Dogbreed2" when "Dog" is selected at dropdown1, whereas you want dropdown2 do display "Catbreed1" and "Catbreed2" when "Cat" is selected at dropdown1.
But do you really want to reload the site after selecting the first dropdown?!
What you are trying to achieve has to be handled by the frontend of your application and is not django/backend related. This does not require reloading.
One way of doing it would be to simply add both dropdowns, catbreeds and dogbreeds, to the html and then change its visibility with CSS depending on the choice of dropdown1 utilizing javascripts onchange event handler.