the original name in Hassania "Trouk" can also be found on French maps.

The Spanish "Isla Herne", on the other hand, seems to go back to the first colonizer Hanno, "the sailor" in 470 BC.

https://ceb.wikipedia.org/wiki/Isla_Herne

the naming "Dragon Island" is a tourist nickname. Here is an assumption that the coastline of the island resembles a dragon when viewed over it. 
More credible is its derivation from "kite surfing", (kite = dragon) the popular sport that, in the former times of campers and kite surfers until 2020, shaped the earlier image of the Dahkla Lagoone with countless colorful kite flyers.