Jodie Scotland-Turner was in training to ride the three-year-old called Morris at a series of events this year, including the showpiece at Ingliston in June.
But the youngster will no longer be able to enter Morris in the contests after part of his mane was severed during an attack in his field.
Scottish SPCA chiefs said the bizarre incident may have been an “act of sabotage” by a jealous pony owner, or even someone involved in Satanic rites.
Jodie’s grandfather, Malcolm Scotland, downplayed the likelihood of sabotage but said his granddaughter was disappointed not to be able to ride Morris in show contests.
The 55-year-old said it would take around eight months for the forelock to grow back and allow the Welsh Section A mountain pony to compete.
The attack was carried out in a paddock in Drumshoreland Road in Pumpherston, West Lothian, sometime during the day on Monday.
Mr Scotland, who runs a dog grooming business, said: “Morris was bought last year so my granddaughter could show him. Jodie has been working to break him in and getting him under the saddle ready for events, including the Royal Highland Show.
“It’s very bizarre that this has happened. It looks like someone had used scissors because it’s a very neat cut. It doesn’t look like it’s been hacked off with a knife or anything. Whoever did it seems to have come prepared.
“I’ve heard of cases where thieves will plait a horse’s forelock so they can go back later and know which one to steal. Maybe that was a version of what happened here. But this is a show pony so cutting off the forelock would be pointless.
“I don’t think it would be anyone else taking part in competitions. It’s not cut-throat enough for that.”
His granddaughter has won the Scottish Welsh Pony & Cob Association’s points championships for the last two years, and was looking forward to competing with Morris this year. Competitors have to demonstrate their riding skills along with showing the grooming and condition of their pony.
Mr Scotland, from Winchburgh in West Lothian, said: “She’s disappointed but fortunately she has two older ponies to ride. Morris was one for the future and now she’ll have to wait until next year. They have to be shown in their natural state so he needs his forelock.”
Scottish SPCA chief superintendent Mike Flynn said: “There are a number of reasons why this pony’s forelock may have been cut and, while this would not have caused the animal any physical harm, it is likely to have been frightened.
“This may well be an act of sabotage by another owner to stop this pony competing.
“It could also be a sign that someone intends to steal this animal as thieves will often cut or plait the manes of the horses or ponies they want to steal so when they return with transport their target is easily identified.
“Incidents such as these have also been suggested to be linked with Satanic practices.”
A second suspicious incident involving horses occurred between 8pm on Monday and 9am on Tuesday at the West Lothian Riding for the Disabled venue. Staff at the centre, near Abercorn, returned to find the padlock on the gate had been forced off.
A police spokesman said: “Those who live in the country are asked to be vigilant and if they witness any criminal activity to contact police.”