Great-grandfather Stephen Bamford, 60, suffered chest and abdominal injuries after being struck by the vehicle on May 29 last year.
Mr Bamford was best known for setting up Thornberry Animal Sanctuary in Rotherham in 1988 and helped to rescue and rehome thousands of animals over the course of 20 years.
An inquest held at Chesterfield Coroner's Court heard how Mr Bamford had been cleaning out horses in the stables with civil partner Peter Revill and friend Paul Spencer on the morning of his death.
Giving evidence, Mr Revill described going to get some chewing gum from the house at Meadow View Stables when he heard a "horrible scream".
He then saw a horsebox lorry had gone through a fence and was travelling across the field.
Mr Revill said: "I ran to the lorry as fast as I could, jumped in, put my foot on the break and managed to stop it.
"That's when I saw Stephen lying on the ground."
Mr Bamford - who ran a livery and shop from his home address - was put in the recovery position but he was tragically pronounced dead shortly before 10.45am.
An investigation found the horsebox didn't have any defects and it was concluded that there was no evidence of suspicious circumstances, an inquest heard.
But in a written statement, Mr Bamford's daughter, Louise Medlock, 44, told the inquest: "I'm concerned this may not have been an accident."
Coroner Emma Serrano recorded a conclusion of accidental death.
She said: "From the police evidence, it seems to me Mr Bamford was stood near a fence post and the horsebox moved and its open-door hit him, causing the injuries he sustained.
"The horsebox continued to move."
His devastated daughter Louise, who never got to say goodbye to her dad, paid tribute to the "devoted" family man who would help anyone in need.
The mother-of-two, who lives in Rotherham said: "I had a fantastic relationship with him. He was just a lovely, caring man and a lot of people loved him.
"He would help anybody in need, he was there for people. He also loved his horses and dogs, that's what he lived for.
"Since he died it's been very hard and there has been a lot of heartache. It's made even more upsetting because I never got to say ta-ra.
"I did manage to give him one last ride in a carriage at his funeral. His favourite colour is yellow so I got him a yellow coffin. I made sure it was a celebration of life.
"But he should be here. It's still heartbreaking to this day."
Recalling the day her father died, support worker Louise described receiving a phone call to say her Mr Bamford had been involved in an accident at the farm."I asked 'is he at home or the hospital?' and was told that he was still at home," she said.
"I thought he must have been kicked by a horse. I never thought that he would be dead already.
"I took my grandson to his mum's and went across to my dad's which is about a 40-minute drive.
"I pulled up was left shocked to see up to nine police officers, two crime scene incident vans and tape across the middle of the driveway.
"I got out of my car, ran into the house and shouted 'where is my dad?'. A police officer took me outside and said 'sorry to tell you but your dad's had an accident and had passed away'.
"I didn't believe he was dead. I went over to him in the field and gave him a kiss on the forehead. You could see he had been crushed on one side of his body."