So my memory was hazy enough I thought the pets lived in George and Mary's house - not with Billy (though they seemed to spend much of their time over at the bank)! Eep.

Fun fact: the crow got more screen time than most of us ever will:
https://en.wikipedia.org/wiki/Jimmy_the_raven

And all pets kept by Billy just seem to be because he was an animal lover; there seems no official word otherwise on why he had such a collection.