Crafty
You mentioned a dog in your OP. You can and should get a harness for your dog from your local petshop, they come in all sizes and have a handle on them for attaching to a seatbelt. Unless my dog is in my Landrover behind a dog guard she is in her harness attached to a seat belt.
In an accident all untied items, including dogs, become missiles in the car. My Jack Russell which weighs about 8 Kilos would have the force of about a ton in a 30 mph accident.
It annoys me to see so called 'dog lovers' with their dog running around the back of a car loose for the sake of a £10 harness.
Its not only you who would be hurt by an accelerating dog but also the dog!!