The lurcher was hurled from the moving truck on the A52 in Radcliffe-on-Trent, Nottinghamshire, at around 2.45pm on Wednesday. Witness Chris Hallam said he was travelling along the road when he saw something “flying through the air”.
Mr Hallam stopped his vehicle and rushed over to help the dog which was lying by the road.
He put the animal into the boot of his car and took him to Minster Vets in Nottingham.
But the dog, called Ice, had to be put down due to his injuries.
Mr Hallam said: “It was just the most horrific thing I have ever seen, the poor thing didn’t stand a chance.
“I was driving along the road when I noticed something flying through the air as I passed an old farmer-type pick-up truck coming in the opposite direction.
“I pulled into a lay-by and ran over to help and two other guys who also stopped said they had seen a commotion in the back seat just as it happened.
“So we’re pretty sure it was deliberately thrown and just don’t understand how anyone could want to do that to a dog.
“The noise coming from him was heartbreaking, he was in so much pain so I took him to the RSPCA at first.
“But they said they couldn’t do anything so I took him to a vets and they came out to my car and sedated him and put him on a stretcher.
“I went to work and my mum phoned me later on to say it hadn’t survived.
“He had suffered broken bones in his pelvis and internal injuries which they couldn’t operate on.
“I was devastated because if he had lived we were going to provide him with his forever home.
“We know his name was Ice from the microchip but the dodgy thing was when the vets called up the name on the chip, they just said ‘oh I’ve sold that dog’.
“I’m convinced this was done on purpose. If it was an accident you would know something had happened and you would stop.
“You wouldn’t drive off and leave him lying there on the side of the road to die.”
A spokeswoman from the vets added: “He was just so terrified and in a lot of pain.
“We had to sedate him while in the back of the car before we could take him out to examine him, which is when the scans showed the problems of the internal injuries.”
Nottinghamshire Police have launched an investigation and are appealing for anyone with information to get in touch.
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Inspector Rob Lawton said: “Police were sadly made aware that a member of the public had been travelling along the A52, Whatton, at around 2.45pm on Wednesday (20 October 2021) when they witnessed a dog falling from a vehicle.
“The vehicle failed to stop and a member of the public stopped and recovered the badly injured dog.
“I am deeply saddened to hear that, after being taken to a local vets, the dog lost their life.
“The welfare of our animals is something I take very seriously and is deeply important to me. Likewise, I know is important to the Rushcliffe community.
“Whilst the vehicle is yet to be traced, we will of course review any new or further information provided to us.
“As an owner of three dogs, I know how important it is to understand the responsibilities involved in protecting and owning pets, including keeping your dog secure.
“I would genuinely like to thank the member of the public and veterinary team for their efforts in trying to save the dog.
“If you have any information or dashcam footage relating to the incident, please get in touch with us on 101 quoting incident number 398 of Wednesday 20 October 2021.”