Oops, read this again. If it's only a small part of a nylon collar, mineral oil MAY work.

However, with a confirmed leash-swallow, or a long enough section of collar, by the time your dog is vomiting profusely, you need surgery, and without it your pup will die.

When a dog FIRST swallows something like that, you can give it 5 ml of hydrogen peroxide per 10 lb of bodyweight. This will make the animal vomit-out whatever it ate because it is still in the STOMACH.

However, AFTER the animal has had whatever it swallowed inside it for awhile, the object then passes into the INTESTINES and can no longer be vomited back out. Yes, sometimes mineral oil will help an animal pass certain foreign items that it swallows, if small enough. However, with swallowing a leash or a rope, or any odd-shaped hard matter large enough, the animal is far more likely to develop an intussusception, which is a life-threatening kind of blockage that will kill the animal without surgical intervention.

Been down this road many times, and what you need is a vet, and ASAP.

Jack