The horse, named Harry, was found on Saturday impaled by a fence post in Holywell, north Wales, having apparently attempted to escape his enclosure having been spooked by the sound of nearby pyrotechnics.
RSPCA inspector Jenny Anderton said: "The horse must have been laying there all night and it took us until 1pm to get him out, get him up and get him on a wagon for the vets to attempt surgery on his very serious wounds, caused by impaling himself on the hedge and fencing."
"We of course don’t know for definite, but it seems pretty likely that Harry was frightened by nearby fireworks that were set off," she continued.
The horse was eventually freed by his owner and a vet with the help of two RSPCA officers.
Lily Roberts, the owner of the stables where Harry was kept, said she had "never seen anything like" it.
She said: "He had crushed the hedge and we had to cut the hedge around him. It was just horrible."
The RSPCA said it received 400 calls about animals affected by fireworks in England and Wales last year, and has launched a campaign, Bang Out Of Order, to campaign on the issue.
Among the measures it wants to see implemented are noise restrictions on publicly available fireworks, and restrictions which see their sale limited to the time around Bonfire Night, New Year’s Eve, Chinese New Year and Diwali.
Ms Anderton added: "We really hope Harry recovers - it is just so sad to think how frightened he must have been and then in so much distress while he was impaled.
"This incident really does show what the effects of fireworks can be and we urge people to be mindful of the animals around them when setting fireworks off."