Irish Sports and Draught cross Tyersal Hill Clover was found with his lock missing in Old Hall Stables, Timothy Lane, Batley, on May 18 by owner Andrea Hibbert’s son William.
Just the day before, family friend Beth Howley, 18, triumphed in all her classes, including Ridden Championship, during a show hosted by Castleford Riding Club.
Beth’s mum Kailey Woods, of Windsor Road in Howden Clough, said: “The horse hates his forelock being touched so it must have taken a few people to do this. The horse is distressed and so are all the family.”
Andrea, of Dark Lane, Batley, said that in horse shows the animal’s appearance is an important part of the judging process.
She said: “It’s the fact that we’ve taken a step up and we’ve got to bigger shows.
“And now it’s going to take forever for that hair to grow back.”
Andrea said that because Beth, William, Kailey and herself were so pleased with Sunday’s results, they posted about it on Facebook.
They believe someone who has seen this may have carried out the attack on the horse, because Tyersal Hill Clover was the only horse at the stables targeted.
She said: “I can’t understand why people are so jealous. Everybody loves him, everybody around Batley knows him because he’s so gentle.
“Beth’s done really well and somebody’s just stopped that.”
It was another blow for Beth, whose previous pony, Missy, became lame and could not be ridden anymore.
Kailey said: “Me and my family work very hard and work long hours to enable us to keep our two horses.”
“Beth is only 18 and worked very hard all year so she could come out and compete.”
It was only because William offered her the use of his horse that she was able to compete.
A West Yorkshire Police spokeswoman said:“The offence is reported to have happened sometime between 6.30pm on May 18 and 10.30am on May 19. At this stage there are no further lines of enquiry and the matter has been filed pending any new information coming to light.”