A disguise is an item, object, or spell that changes a character's appearance beyond what the game typically allows, normally found in the form of buffs and debuffs. Sometimes being in a certain zone or phase of a zone will change your character into something else. Getting your hair cut or colored at a barber would not be a disguise. An effect on the character that does not substantially change the character's appearance would not be a disguise.
Implicit is the assumption that being in disguise is voluntary, and the player retains control of the character. A hostile transformation whose main purpose is to cause the loss of control of your character and not to in any conceptual sense provide a deceptive appearance is not considered to be a disguise, however, it might be exploited as a disguise by a creative team in unusual circumstances. After all, sheep critters, polymorphed player sheep, and explosive sheep all look pretty much alike.
These are disguises which take up inventory space. If you use them then the disguise will be applied to you or your target. Most of the ones which apply a disguise to a target require you to be partied with that target. Certain ones are consumed when used.
Engineers can use their teleporters, [Ultrasafe Transporter: Toshley's Station] and [Dimensional Ripper - Area 52], in an attempt to disguise themselves with a debuff that lasts an hour. Unlike other disguises, these can stack if you logout then log back in after 4 hours, but people prefer just to have fun with the malfunctions while they have them.
Turns your mount into a Brewfest ram or kodo, depending on your faction.
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Quest disguises
These only last until the quest they belong to are completed. Most are limited to a certain zone. [28] Raene's Cleansing (6) is one of the only quests that you can abandon and keep the item forever, but you can never finish the quest or [Dartol's Rod of Transformation] will be removed from your inventory. [Overseer Disguise] can be made continuously, but it's limited to its quest's zone.
Shadow priests can use Shadowform to turn themselves a dark purplish color. Some may argue that this is not a disguise, however at one point it use to share the same blanket effect as disguises. That means it actually removed disguises once.
NPC spell disguises
If you interact with certain NPCs, they can provide you with a disguise. While there are a few NPCs that will give you a disguise just by speaking to them, a majority of NPCs will only cast spells on you for certain quests until they are complete.
The Pirates' Day buffs alter only the appearance of the character's clothes, but are disguises in the sense that they bypass the normal clothes mechanic of having to equip the items to display them and overide the appearance of equipped items. The buff can be obtained through talking with one of the commoners that spawn in the capital cities. You can get a longer lasting buff from Dread Captain DeMeza who's in Booty Bay during the event.
During Hallow's End, sometimes innkeepers will trick you instead of giving you a [Treat Bag] by turning you into mini diablo, a cat, a miniature sand worm, or a frog. The debuff from being tricked only lasts for a short period of time, but some players like to run around with it.
Bom'bay <Witch Doctor in Training> provides several short term spells; one enlarges the character, and one makes it a "ghost" - makes it more transparent.
Using the item [Brazier of Dancing Flames] and then selecting the NPC that spawns, Dancing Flames, will allow you to interact with her through some emotes. The emote /dance will turn you into an enlarged version of her.
Zone disguises
When you step into certain zones, your character will become disguised.
Storm Peaks has some subzones where you turn into a female frost vrykul. The subzones will only turn you into a vrykul if you're on certain quests or have completed them.
Equipment disguises
Equipped clothing items, which are intended to change appearance, could be counted as disguises. However, the mechanic of equipping armor is not uniquely beyond what WoW normally allows to change appearance. Allowing clothing items to be called disguises leaves every costume item open to debate, however, not allowing this still leads to ambiguities.
[Elixir of Giant Growth] and [Deviate Fish] (one effect is shrink) are on the periphery as they change the character's size but none of the graphic elements.
You can trick the disguise buffs into thinking you're your opposite gender! All you need is [Pygmy Oil]. Make sure you drink enough to turn into the gnome, then use any other disguise which adjusts to genders. The disguise you use must have a duration lasting over what's left of the [Pygmy Oil]'s buff. For example, if your disguise duration is 5 minutes long, you must wait until the oil buff duration reaches 5 minutes or below.
Patch changes
Patch 1.7.0 (13-Sep-2005): When you have more than one transformation aura (Orb of Deception, Noggenfogger, etc.), the last one applied is the one that is shown.