Red fox, as that is the only species we have here in the Netherlands. Anyway, I at least assume it is a fox. Except for a domesticated dog left to die in the dune reserve, I can't think of any other wildlife species here in the Netherlands that would have such a skull.
I don't mean to get too far afield on this, but this is clearly not a fox's skull. Foxes are carnivores, which means they have prominent (and pointed) canine teeth and other teeth meant to cut and tear flesh. This skull is not that of a carnivore, but rather that of a herbivore or grazer, as its teeth are suited to chewing plant material. Not sure what it is but it is clearly not a deer either.
Here is a photo I found online of a red fox's skull, notice the canines top and bottom and the sharp pointed molars:
Stm is right, I corrected my mistake. It is most likely, or almost certainly a female roe-deer skull. Front part of the skull (nose) and lower jaw of course missing