From Guild Wars Wiki
Skill Type. A
Disguise always changes the appearance of characters, using a new model, just as
Forms.
There are two types of disguises, those that only changes apperance and those that also alters the abilities of characters into that of a different creature. The latter may involve new skill bars, max health, max energy, attributes, and energy regeneration.
An example of a Disguise is seen during Nightfall's Blacktide Den mission, where characters are disguised as Corsairs in order to advance the plotline.
[edit] Disguises obtained during missions
[edit] Disguises obtained in explorable areas
NOTE: Desert Wurm is labelled as a disguise, but functions like a form in that it changes the user's attributes and abilities. Inconsistently, Siege Devourer, which behaves similarly, is considered a form.
[edit] Disguises obtained by using items
[edit] Disguises obtained during holiday events