Mr McKenna believed the owners of the north-bound vehicle and float had travelled from Wellington.
"One horse had fallen and was stuck under some steel railing. It was very frightened and was kicking and carrying on as it lay there.
"It took us a couple of hours to get it out after waiting for a vet to arrive to calm it down."
The owner had stationed two firefighters from Featherston beside the neck and shoulders of the trapped horse to calm the animal and keep it from attempting to stand, and risking injury to itself and its rescuers.
"The owners were very concerned about their animal and were very upset, which is understandable. We had three people lying on top of it, the owner of course, and two of our women from the brigade while we waited for the vet.
"Every now and then it would kick and take a piece out of the side of the float, which didn't look too good by the time we were finished. We were having to be very careful with the traffic going past too, because it was right on the side of the road," he said.
"People were going over to the rugby in Wellington and we were holding them up, so it was a really dangerous situation as far as I was concerned."
The veterinarian arrived and sedated the horse, Mr McKenna said, and "we'd got Greytown to come down with their jaws of life, and they cut the steel pieces out".
He said there was no injury to the horse or any of its rescuers but the animal had caused significant damage to the float during its ordeal, which was "pretty rare".
"In my 40 years in the brigade its only the second one I've been to where a horse was stuck in a horse float."
Greytown firefighter Steve Perry said he was surprised by an apparent show of gratitude from the horse after it was led from the float.
"Once he was free and standing we expected he was going to bolt. But he went straight to a Featherston firefighter and gave them a bit of a nuzzle, as if to say thanks."