Staff, clients and pets escaped without injury when the buck came through Family Pet Hospital in Mount Olive Township Tuesday.
Dr. Sue Bliesath told The Daily Record of Parsippany everyone thought a car had crashed into the building.
The doctor says everyone moved to safety in an exam room while a nurse called police.
Mr. Bailey, a West Ireland Terrier, hid behind a desk in the reception area and was not injured.
The doctor says the deer was very agitated and unable to walk. Police shot and killed the animal.
The doctor says she's fairly confident it would not have survived.