Wildlife rescuers responding to a call about an injured deer in Stanhorn Grove, Blakes Mead, had been told the animal was near a building site.
Once they had arrived they said they were told the deer had got up and run away but it was later found without a head surrounded by knives.
West Sussex Wildlife Protection has condemned the beheading and said it believed the deer was to be taken home and eaten.
A spokesman for the organisation said: “This is disgusting behaviour towards an injured deer. We will never know if this deer was alive when it was beheaded, the time line for this incident does not ring true.
“Deer can suffer stress myopathy and also concussion, we believe it had been in a collision with a car, but after a few days deer can recover.
“These men were not vets and had no right to make a decision on this animals life.
“If people find injured deer they should phone the Police who have a list of people to work from. We also deal with a large number of deer yearly and this has been the worst so far.”