Yes, it is legal and knackermen sometimes do just that when an owner wants to dispose of a carcase. What is not legal is to put such a carcase into the human food chain unless it has been reared and despatched following the laws concerning human consumption meat. BTW there was a big local furore earlier this year when a speciality deli sowner started selling salami made from donkeys...all quite legal but I think that local public opinion made him reconsider