"A lot of people will think of it as a joke. It's not a joke to horsey people."
Mrs Nickolls said she was "just speechless" after seeing the damage.
"I can't believe someone would do that, to be honest.
"They all had long, thick tails - one of them was extra long and they've hacked it off.
"None of them are neat cuts, but they've hacked them off right to the bottom of the bone.
"I don't even understand how, or why, someone would do that."
Initially, the women thought the tails had been taken to form "false tails" on dressage horses - thicker tails in a competition could earn a horse more points.
However, all three tails were discovered in the paddock yesterday afternoon, making the women feel the offending was malicious.
"That just makes it worse," Mrs Nickolls said.
It would take at least three or four years for the horses' tails to grow back and there was a chance Cabby could not be entered in dressage at a national level with a false tail.
Debbie Barker, of Auckland Horse Haven and New Zealand National Register of Stolen and Missing Horses, said reports of horse tail removals had been received from Christchurch and Nelson in the past month.
Unfortunately, for horse owners, there was little they could do, she said.
Earlier this year, Mrs Nickolls, who is largely self-taught, won a national incentive award for dressage and was sixth in New Zealand at the national dressage championships in level five.
She also moved up to level six/seven, the grade below Olympic level, or grand prix and was also named the high points prize winner for the "advanced" category (level six/seven) in Southland.