No it is not a custom or 'fake' JBD. in fact it is 498, JBDs have no markings when they hatch though.
...Something curious about JBD eggs is that the egg shell's color matches the fur color of the baby it keeps inside. A baby JBD is born with no markings, though a week after it hatches, it will start changing fur, and its markings will appear (unless it turns out that it is Plain)...
JBDs (c) Hiccup
