Alisha Novotny of Belgrade said she entered the pen of her horse, Raz, Oct. 2 and immediately noticed the mare's tail was clipped, KECI-TV, Missoula, reported Monday.
"I just happened to glance at her tail and I was like, 'Oh my gosh!'" she said. "It looks like they just grabbed pretty fast and chopped."
Novotny said the incident left her feeling "violated."
"I felt really weird for three days, just because someone was here and I don't know what they did to her," Alisha said.
Police said thefts of horse hair, which can be valuable for creating bridles and other items, have been an increasing problem in the area during the past half year.