"IS this possible ? "
Anyone can file suit the bigger question is would you win? Probably not. First your dog was not permanently injured. Without permanent injuries there is no case. Second: the law views animals as property regardless of how attached emotionally their owners are.
"Get this, there are mass law suits for humans given reglan !"
Has no bearing on your situation because you are talking about it being prescribed for a dog and the dog wasn't harmed by the drug.
"The new vet said not one of the six vets at her office likes to prescribe it because its an old school medicine"
That doesn't make it malpractice because other vets do use it.
"I cant believe he was afraid of a 9 pound dog!"
If your dog was not feeling well or in pain when the first vet saw him then that could explain the need for a muzzle then but not at the second vet when he was already doing better. A dog that is ill or in pain has a tendency to bite. Even though that particular breed is small their bite is quite severe.
"That's just my opinion, then again I might be wrong." Dennis Miller