Generally only with animals that are not seen as individuals, like insects, or if the gender is unknown. With dogs (and other animals that are typically kept as pets), people will almost always use 'he' or 'she' - and will usually default to 'he' if the gender is unknown.
I don't know what you are talking about. Native English speaker here and everyone uses the pet's gender when speaking about them. Some people make it a point to say 'it' when they don't like animals or they want to insult the owner.