There's not really a story behind the dragon as far as I know, he guards the treasure chest below him which is normally not tied up but open and filled with gold. there is a bracelet or crown in there that you can try to push if you dare, it will make more noise if you do I think.
Here's a short video of the attraction.
https://www.youtube.com/watch?v=NHAbCPLjeD8The entire park is filled with such electronic creatures and many fairytales. Its a very beautifull theme park and the dragon is one of its most iconic attractions.
It's strongly thematic and filled with great innocent fun for children and the whole family.
Opening in 1952, it's one of the first theme parks and always kept expanding.
I'd go as far as to recommend anyone visiting the Netherlands to pay the park a visit. It's truly a magical place.