Six miniature show horses have been found slaughtered in a stable at Clayton Bay, about 70 kilometres south of Adelaide.
Police describe the incident as ''bizarre and cruel''.
Three other miniature horses in the same stable were not harmed.
The distraught owner of the horses reported the incident to police on Saturday.
The owner's son, Daniel Spong, told ABC Radio it may have been a targeted attack but it was not known why.
''It just would take an extremely sick individual, I think, to carry this out. We just can't believe it.''
Police say the horses are believed to have been killed some time between Friday night and Saturday morning at a property at Clayton Bay.