You know, I was going to make this nice and detailed and eloquent, but this has been sititng in my drafts exactly as it is now since I started this blog. So I give up- it’s getting posted as-is and maybe I’ll add some illustrations later if I have time.
Kosomot has a total of four forms he is capable of taking, each having a very different appearance from the others. He can shift his shape no further than each of these forms, however, as that was among the powers of a Maia that were lost to him when he became corrupted.
Mannish - (humanoid, red-orange ombre hair, orange eyes, dark skin, otherwise normal looking, no horns etc. Most common when not in Angband, or when reborn.) Height can change somewhat- will be on the shorter end in reborn verses while taller in first age verses.
Natural - (tall, hair is like fire, tail w/ flaming end, horns, glowing eyes w/ black sclera, VERY hot skin. Most common form in Angband/during the first age.)
Balrog - (traditional idea of a balrog. Massive, wings, made of rock and lava and smoke, big horns, monsterous face, etc. Usually only taken for battle.)
Bestial - (looks like a cross between a wolf and a lion, otherwise much like the balrog form without as much lava. Very rarely taken.)
Why can he shape shift? It’s commonly assumed/noted that Maiar can change forms- and as corrupted Maiar, Balrogs also have that capability, though to a lesser extent, and the forms they can take are usually more limited.