Owning a pet dog
Conventional wisdom has it that Jews don't much like dogs. Some put this down to bad historical experiences.
The Talmud bans owning a "bad dog" in the same breath as owning a bad ladder (Yerushalmi, Trumot, 88:3). Both are accidents waiting to happen. The Talmud also warns that a bad dog "keeps kindness away from your home", as poor people will be afraid to knock on your door to ask for help or charity.
The Rambam says that all dogs in a town must be tied up. By the 16th century the Rema says that since it has become a regular practice to have a dog, it is permitted.
Rabbi Jacob Emden (18th century) allows owning one dog - and more if strictly necessary - but frowns on owning many dogs, "especially those smooth, naked, hairless dogs that people buy for a lot of money for amusement," implying there is something frivolous about having a pet. Guide dogs, of course, are another matter.