A Silver Vauxhall Astra has been spotted in the Thorner are of Leeds on March 7, police said.
The force warned horse owners to 'be aware' after the 'suspicious' reports.
Horse owners have been asked to consider using property marking equipment to identify their property.
Police said owners should then take pictures of their items and include any serial numbers or identifying marks.
West Yorkshire Police - Wildlife & Rural Crime social media team posted: "Horse owners be aware.
"We have had a report of a suspicious vehicle been seen around the Thorner area of Leeds on the 7th of March 2021.
"The vehicle involved was a Silver Vauxhall Astra. Please be vigilant and make sure that your tack and equipment is secure and safe.
"Consider using property marking equipment to identify your property and take some pictures of it and include any serial numbers or identifying marks."