Jake, the Great Dane, was sweet as pie and I was careless in patting his backside as he was being led out by his owner after a dental. He was still dopey, so he whirled around and snapped at my face in surprise. I didn't realize how close he'd gotten until I noticed my gold necklace lying broken on the ground. He'd actually grabbed it off my neck! *shudder* I learned after that not to be so careless, even if the dogs are familiar with me and normally very friendly.
The other was Cain, a Rottweiler who was a product of years of cross breeding and had a serious mental disorder. He hated all men and was OK around women. Kids were a definite no-no! Yet, his owners (who had 2 young children) wanted to "cure" him, despite our warnings that it was not a temporary or treatable condition.
Long story short, in a stupid act of bravado, I volunteered to get Cain out of the kennel to bring him up to the front where his owners were waiting in a exam room. They handed me his halter (not a leash and collar, folks - a HALTER! It goes around the body and secures underneath the animal.) I got Cain out and secured the halter, his tail wagged the whole time. As soon as I went to get up from the crouch position, he went stiff and started trembling. I knew I was in deep trouble. I slowly lifted up, leaning away from him and avoided eye contact, but it was too late.
He sprung at me, snarling and foaming and lemme tell ya - I was fit to die right then and there. He wasn't a year old yet, but he weighed 89 pounds and was an unaltered male. YIKES! I ended up with my back against the wall, yelling for help and just started punching him in the face everytime he made a leap for my face. I actually broke a knuckle and didn't realize it. Cain finally grabbed a hold of my right forearm and shook it a bit, but he was slowing down already. I took my opportunity right there and kicked him in the ribs, sending him sliding across the cement floor and gaining me enough time to scramble out the door.
The vet had arrived by then and got the dummy stick (as we call it - it's actually a noose on a pole) and caught Cain. I wrapped my arm and sat down, shaking and crying. I saw the vet lead Cain to the exam room where the owners were waiting and guess what? Cain attacked them! As soon as the door opened, he went for the husband. AMAZING! The vet apologized and said that they could no longer bring Cain to our office because he had proven to be a threat to the staff.
And the most unbelievable part of the story is - last I heard, they kept the dog!
Some owners are truly stupid, but in the case you mentioned, it was probably the dog reacting from pain and fear.
Try the same thing with a psychotic, unsocialized Arabian stallion. It's like living with an unexploded 2000 lb. bomb, and I know some people that do that (the horse, not the bomb).
Before getting her current stallion as a youngster (a real sweetie with the personality of a Golden Retriever), Marl leased a near-feral stallion from a "friend". He was a good looking horse, but spent his entire life alone in a field, with almost no contact with people or other horses. He produced good-looking, intelligent, lazy kids who had his trait of avoiding work at all costs. At least they were just crafty and stubborn, and not dangerous.
He, OTOH, was so dangerous that Marl would keep him in a stall all day, and only turn him out at night, alone. She was afraid for any kids who might take a shortcut through his field. Even I, thankfully, was not allowed to get near him.
One morning, Marl was in his stall, giving him a bucket of fresh water. As she bent over, he decided to take a chunk out of her back. He didn't break the skin, because it was winter, and she had about eight layers of clothes on, but she had a huge black-and-blue mark for weeks.
She spent the next 30 minutes expressing her displeasure by beating him with a lead line. Then she called her "friend", and told her to get her damn stallion before I got there the next day and shot him for her. The horse was gone in two hours, and we haven't heard a thing about horse or owner since.
I arrived the next day, and heard the whole story as we crept through a snowstorm to look at a colt she had gotten a tip on. Baby Aki stole our hearts, and 12 years later, he seems like the kind of horse an elf would raise. He was destined to be someone's dream horse, and that someone was Marl, rather than the woman who bred him, but died before he was born. He's the quietest, sanest, hardest-working, most "together" horse I've ever worked with. I've seen him in situations that sound like something out of a fantasy story, but it's really him.
He's the poster boy for good breeding, strict upbringing (including socialization by a grumpy old gelding), lots of love, and plenty of mental challenges. He's always happy, except when out in the rain, where he acts thoroughly miserable.