Most probably it died in one of the bottom set gill nets which are deployed in shallow water. Recreational fishermen set these nets on the beach and in the shallow sea at low tide. Harbour porpoises are the smallest of dolphins occurring in the North Sea. They used to be common along the shores of Belgium up to the 1960 ies. It is a threatened species.
Most common threats are pollution, disturbance and bycatch in fishing gear.
The animal will be thoroughly investigated by specialists of different universities. The pictures show the animal and a detail of a net marking on its snout.