Residents had spotted Stella being ridden along Napier roads, including Marine Parade.
She was found dehydrated and unable to walk, with a swollen mouth and ulcers, tight muscles and tendon swelling in her legs, as well as damage to her hooves.
A vet described the pain as a person walking without toenails.
Ashleigh was pleased with Obrien's plea yesterday.
"I am so over the moon about it; just so happy that he's said yes, that he did do it. They definitely deserve time [in jail]."
Ashleigh said while Stella's condition had improved markedly, she was still very sore and on some days required painkillers.
Ashleigh's mother, Sharon Steed, said news that Obrien had been charged and pleaded guilty to taking and cruelly mistreating Stella was "brilliant" and a "victory", after a long police search for the offenders. "There were three people riding her and thrashing her around Napier. It's a victory for us and all animal lovers, it's something they just can't get away with."
A police spokeswoman said a total of three people have been arrested in connection with the ill-treatment and unlawful taking of Stella.
Ms Steed said Stella, who was an SPCA case before Ashleigh took care for her, was also beaten on the head and was now wary of people touching her after months were spent gaining her trust.
"We've started to build her trust back again, but we couldn't touch her head for a while. It took her quite a long time to get over that fear. I don't think the foot is ever going to be right again though.
"Those people who took her have no empathy at all, they thought it was funny but that horse is still suffering today because of what they did."
People wanting to follow Stella's road to recovery can be updated via a Facebook page called "Stella's Story".