I second GSD in the mix.
Considering the whole Island is only the size of Albuquerque, and has typical island import restrictions. how likely would a person that had a purebred Malinois or other even less common breed that would have to be imported to the island (which the vetting includes mandatory microchip) just running loose to breed puppies with the native Cunucu population? It might be wholly native dog.
When thinking of the location he was born. Aruba has a very proactive stray catching program for some years and a draconian time limit once in custody. They also have some heavy import restriction though no true bans noticed other than Airline restrictions(I couldn't bring my pets at the time -40lb chow and a 20 lb persian- snub nose animal breeds)