Two other horses in the Lake Terrace Rd paddock also suffered also minor injuries and have since been moved.
Owner Karina Romeyn was beside herself with the news her beloved pet had suffered such a brutal attack.
"I was almost ready to give up on the world. But the messages of support and offers of help have been overwhelming," she said.
She said a vet has offered to look after the other two horses medical needs until they are fighting fit again.
"It is absolutely amazing," she said.
Mrs Romeyn bought Gus about eight months ago to help her "get back on the horse".
"I had a big fall [off another horse] in 2014, where I broke my pelvis, back and neck . . . Gus was helping me recover. Every time I was down, or in pain, I would go and hug him and it would be alright," she said.
She had taken him for a few rides after having two surgeries to repair her neck.
Her children, Monet, 17, and Loughton, 16, and niece Claudia Anstice, 21, are all traumatised by Gus's death.
He was such a lovely natured animal, Mrs Romeyn said.
"He would be the first to the gate and was so inquisitive. He didn't deserve this," she said.
Anyone with information, phone 363 7400 or anonymously through Crimestoppers on 0800 555 111.