"We were hopeful that the foal could have recovered, but support the veterinarian's decision and know it was the only one possible given the circumstances," Ms Midgen said.
"Unfortunately there are times where despite veterinary treatment, an animal is in pain and suffering, and there is nothing else we, or anyone else, can do to mitigate this. In those circumstances, euthanasia is the best option for that animal."
SPCA inspectors were happy that the needs of the rest of the herd were being met. They would continue to monitor the situation and work with the owner.
Ms Pfaender said she was upset that the animal had to be put down.
"There was one little foal that didn't do well. We tried everything possible, and had a vet involved, and it's now out of its misery. We are thankful for offers to help with this foal, but it just wasn't going to make it," she said.
The remaining nine horses were in good condition, but she would rehome them as soon as possible.
Meanwhile a post on social media attracted a number of negative comments about Ms Pfaender, and the SPCA. Those comments, and the original post, were taken down late last week.
A spokesperson for the NZ Horse Welfare Coalition said the organisation had been made aware of the horses and foal on Wednesday, but understood that members of the public had contacted the SPCA on March 26.
NZHWC had contacted the owner twice to offer financial assistance, medical care and support, and asked her to sign the foal over to it. Those offers were refused. She had been told that the foal was to be euthanased.