Meralygos

Right, since I recalled Kalecgos mate refusing to take human form and instead took a high elven form because of that humans weren't "elegant enough" or something along such lines.

So this made me slightly curious. What does your character disguise as? And most importantly in this topic, why that disguise? Is there any race they'd really not want to disgusie themselves as? You get the idea!

Hmm, an interesting topic. 

Well, Silagos choose an old man - Silandos - for several reasons. For example, the guise of a venerable mage allows the dragon to have a respect and authority. Furthermore his vast knowledge required a fitting individual. Despite being young Silagos is extremely resourceful and taking the shape of a young apprentice with a knowledge capable of rivaling their masters may reveal suspicious. Plus he needed a venerable form through which he may impart his (moral) lessons. At the same time, aged wizards do not require any bond, as they usually have been living long enough. This allows the dragon to go under the guise of Silandos without risking much.

An other form Silagos enjoys is that of Silastar, a blood elf. The elves don't have much to share with the blue dragons but their love for arcane magic - which the dragon finds fitting enough to develop a certain sympathy towards said race. While the whole "blood elf topic" is a bit controversial (he doesn't like them a lot), he is civilized enough to appreciate the majesty of Silvermoon. And of course, after they have taken the Focusing Iris and built up a mana bomb, perhaps he should keep an eye on them!

As for other races, Silagos loathes orcs ("brutes that stole the Iris to wage their war are hopelessly obtuse"), highbornes ("they are the cause of the War of the Ancients and the dragonflight's downfall, even Malygos' ultimate demise! In ten thousand years, they made nothing to redeem themselves!") and of course, the goblins ("deceiving little servants of Deathwing who would sell their mother for a copper"). He has nothing against the taurens and the dwarves (or the night elves), but doesn't find these two interesting enough (or fitting his taste, that is). Beer is not really something this drake enjoys. He likes gnomes: inventive minds capable of great feats are highly valued by Silagos. However, he thought humans may pay little attention to a gnome and became an human himself.

Alygosa used to pose as a Quel'Dorei, but when they chose to become Sin'Dorei, she was angered by their shortsightness, and instead set out for the human kingdom.

There, she decided to pose as Ailisana, a sort of timeless looking ginger woman with pale skin and otherwise good and innocent looks, so that she could infiltrate the human society with more ease. Thus far it is working quite well. She is not often perceived as a threat, and can easily extract the information she needs simply by the flutter of her eyelashes and a few sweet words.

She decided to go with human to begin with, because they are the most common and most integrated people on Azeroth. She needs no excuse to be just about anywhere, as humans are so varied and versatile.