Christopher Black, 17, a member of anti-hunt campaign group West Sussex Wildlife Protection, had gone with his mother Carol Tibbits to the Chiddingfold, Leconfield and Cowdray Hunt's meeting at Coombelands on Friday morning.
They left the Nissan Micra parked and returned later that day to find the windows smashed and the broken body of a fox lying on the passenger seat.
West Sussex Wildlife Protection organises the majority of anti hunt protests in the area.
Spokesman Simon Wild said "We've always had an unwritten rule that we don't damage each other's vehicles.
"We leave our vehicles unattended, but so do the hunt supporters.''
Hunt master of the Chiddingfold, Leconfield and Cowdray Hunt Nick Bamber said he did not believe any member or supporter of the hunt had been responsible for the damage.
A Sussex Police spokesman confirmed the incident had been reported and an investigation was continuing.
Full story in the County Times.
See our South Downs section for more stories from the area.