If you followed his explanation, he "shifted his cloak" (or more simply "shapeshifted") to wyvern once he was in contact with one to copy its form.
I'm going to go a step further and say he "merged" with it some how.
I'm definitely thinking merged. The wyvern Lucian fell on in the previous comic has a chipped right horn, as does the Lucian-wyvern you see here.