The 'X' Dragon is only mentioned in The Incomplete Book of Dragons.
Physical Appearance[]
The Author, Cressida Cowell, named this dragon and illustrated its egg, which is grayish with a distinct white 'X' mark on it. She did not divulge any further information about this dragon.
Appearances[]
The Incomplete Book of Dragons[]
Only the egg of the 'X' Dragon appears, in the chapter about dragon eggs. It is mentioned nowhere else.
