A horse found with a puncture wound in the rib area was deliberately injured, British police believe.
Lincolnshire Police say the horse required emergency treatment from a local vet.
Constable Tim Newell said the horse was in a field off Foxes Low Road in Holbeach when the incident occurred.
At some time between 10am and 6pm on Sunday, it suffered the injury.
An examination of the field has been carried out and there appeared to be no alternative explanation for the injury other than that it was deliberately inflicted.
Newell appealed for witnesses to the incident, and asked anyone who saw anything suspicious in the area on Sunday, or who has any information which may assist the inquiry, to come forward.
Contact Constable Newell by dialling 101 and quoting the incident number 181 of July 1. People can contact Crimestoppers anonymously on 0800 555 111.