"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 foal was drenched and while it was eating grass it would not eat special supplementary feed it was offered. The remaining nine horses were in good condition - however, Ms Pfaender said she would rehome them.
"There is plenty of grass and water where they are on the property but they will be rehomed as soon as possible."
A post on social media had attracted a number of negative comments about Ms Pfaender, who said she had been subjected to unfounded abuse.
SPCA CEO Andrea Midgen said as part of the intensive treatment programme the foal had been closely monitored and received follow-up veterinary care this week.
"Unfortunately when the veterinarian visited this afternoon, the foal's health had significantly deteriorated to a point where the veterinarian had to make the decision to euthanise the foal," Ms Midgen said.
"When we are dealing with such young, vulnerable animals, their health can go downhill very quickly. 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.
"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."
She said the investigation was ongoing and inspectors were happy that the needs of the rest of the herd were being met.
Pauline, of the NZ Horse Welfare Coalition, said the group was made aware of the horses and foal in poor condition on Wednesday but understood concerned members of the public had contacted SPCA on March 26.
The group posted on their Facebook page about the incident and included a photo of the foal.
Pauline, who did not want to give her surname for fear of victimisation, said they had contacted the owner twice to offer financial assistance, medical care and support, and asked her to sign the foal over to them. Those offers were refused.
She was told the foal was to be euthanised.
"We cannot accept this foal will be put down. So, we implore you to make some noise if you feel the same and contact the SPCA on Facebook, and by phone and email to express your concerns about getting help for the herd - as the SPCA have the powers to uplift the foal, who surely deserves a chance at life," the post said.