The strangest thing about how the image of the European witch is that the stereotypical witch didn’t start out as old and that dogs too were associated with witchcraft. Like what I said, this varies between locales. At some point or another, dogs were also commonly linked to witchcraft though it’s relatively more common in Britain than in France especially in the early modern era.
So much so that this may have influenced black dog lore a lot. I could still be wrong about it but given that’s what I got from reading that could explain the wealth of black dog ghost lore in the UK. That black dog lore is so extensively studied in the UK, which also had a substantial number of dogs accused of witchcraft is a profound association.
Keep in mind that some of those dogs accused of witchcraft weren’t black but the association’s great enough to be linked to begin with.