Well, Grizelda overheard some wizards talking about sending the "hounds from Hell" to take care of some project or the other. So, she decided that dogs like that must be good to have, especially if you are a busy witch. Thus, her idea of calling her familiars the "hounds from Hell".

She knows Hell is a place in Michigan, and assumes that they must have great dogs there! But, Michigan was too far away to travel by broomstick, so she just went to the pound and is pretending they are from there.