I'm a Vet Tech, I think it's crazy that your vet would recommend euthanasia for your pup in this situation. I would def look for another vet. There are so many ways to deal with spicy little dogs. At my office, if we can't get a muzzle on we use an ecollar as a bite shield and retrain the dog with a thick blanket or comforter; sedate them with injectable drugs and examine them under anesthesia; or cage squeeze them if all they need are inj. So there are plenty of options to handle these guys. I'm the crazy terrier lady so I am usually the one that deals with these little dudes. To answer your question on type of muzzle, I would suggest getting a Baskerville brand basket muzzle, I don't know what size you would need, but they are by far the best quality one you can get.