This post is a cataloging of all the trinkets, consumables, and suits that make you look like a completely different creature.
None of these items will trick enemies into thinking you're one of them, as these are cosmetic only.
Orb of Deception. [FARMABLE / AUCTION HOUSE] Previously mentioned in its own post, this trinket turns you into another race for 5 minutes with a 30 minute cooldown. You always turn into the very same model, it never changes, and anyone of your race and gender will also turn into your exact same model. The model is not random, nor is it based on your personal character design, nor does it keep your original gear. It is a set look. Races: Gnome/Tauren. Human/Orc. Night Elf/Undead. Dwarf/Troll. Draenei/Blood Elf.
Orb of the Blackwhelp. [VENDOR / CONSUMABLE] 2.5g and 3 charges for each. Turns you into a black whelpling for 15 minutes or until you perform an action / take damage. You must farm a specific gear set in Blades Edge Mountains to buy these from an enemy vendor.
Orb of the Sin'dorei. [FARMABLE] Chance to drop off any boss in Heroic Magister's Terrace. Lasts 5 minutes with a 30 minute cooldown. Will change anyone (except blood elves) into a Blood Elf. Your gear stays the same and your character changes based on its own personal design. So you and your best friend, both human females, can use it and will come out looking like different blood elf women in your old gear.
Time-Lost Figurine. [FARMABLE] 3% drop off of Terokk, a 72 (elite) in the middle of Skettis. Turns you into an arakkoa (bird person) for 5 minutes with a 30 minute cooldown.
Dartol's Rod of Transformation. [ALLIANCE ONLY QUEST] Turns you into a Furbolg for 3 minutes with a 1 minute cooldown, breaks on damage but not on casting, and cannot mount while in use. Start the quest chain Raene's Cleansing until you get and activate the rod. You can drop the quest without losing the item, but if you finish the quest chain (which involves turning in the rod) you'll lose the rod forever.
Noggenfogger Elixir. [QUEST / VENDOR / CONSUMABLE] You have to complete a quest chain for good ol' Marin, but afterward you can buy this off him anytime you want. It gives you a random effect: Slowfall (fall slowly), Shrink (you become smaller), Skeleton (you become a skeleton).
Savory Deviate Delight. [COOKING / AUCTION HOUSE / CONSUMABLE] This turns you into either a pirate or a ninja. There are only 4 character designs available: the male and female versions of each. Everyone looks the same when they use it, but you can cast and take damage with it on (thus it's a favorite raid buff for tauren males). The recipe itself is a fairly rare drop in the Barrens (horde teens area), so it's a lot more common on the horde AH than the alliance AH. Also, to cook these, you have to fish in specific pools in the Barrens. It's a lot of hassle, and I suggest just being a consumer unless you want to make a business out of selling it.
Murloc Costume. [EBAY] Put on a giant murloc suit with a little Blizzcon flag. This was a gift item at the 2007 Blizzcon and can be found on Ebay. If interested in the pricing trends, take a look at WoWTCGLoot. Today it's going for a $195 buyout.
Carved Ogre Idol. [EBAY] Use this trinket to become a big red ogre for 10 minutes with a 10 minute cooldown. It stays on even when you take damage, but the funnest part is the Chris Farley SNL dance. This is a special gift earned by people who play the Trading Card Game (TCG), but some of them are selling theirs. On WoWTCGLoot right now the cheapest is a $69.95 buyout.
Gordok Ogre Suit. [QUEST / CONSUMABLE / CRAFTED] Use this to become a big black ogre. Get the pattern for tailoring or leatherworking from doing this quest. The suit only lasts for 10 minutes (whether you're wearing it or not) but it seems to be tradeable. I'll do some testing.
Hook of the Master Angler. [CONTEST PRIZE] Ever wanted to have druidic swim form? Well, you can! Just win the weekly fishing tournament and one of the two prizes is this trinket to become a fish -- complete with underwater breathing and increased swim speed. Breaks on attack or being attacked. No real cooldown (5 sec).
Winter Veil Disguise Kit. [QUEST / HOLIDAY] Turns you into a cute snowman. Requires snowball to use (bought at seasonal vendor or produced by an engineering item).
Hallowed Wand. [HOLIDAY] Turns you into a special costume for Halloween. Current wands include: Bat, Ghost, Leper Gnome, Ninja, Pirate, Skeleton, Wisp, and Random. Wands are BoP but do not disappear.
Thanks to everyone helping with the ones I missed! Keep them coming. :)