post rumors and theories about this monstrosity here.
feel free to use you imagination.

Ani-Form (also known as heartless sora) is sora becoming ALMOST a complete heartless as, he does not use a key blade and is completely covered in pitch black darkness. As final-form resembles a nobody, anti-form resembles a heartless. Anti-form may be stupid as it can't pick any thing up or use magic and walks n all fours but it can use abilities. (anti-glide etc.) For an unknown reason, using valor and wisdom form raises the percent of anti-form accruing but master does not to any thing to the counter. Final-form lowers that counter for another unknown reason. Anti-form has lowered defense but raised attack and is quite quick.
Anti-form still remains a mystery. I hope its explained in the next kingdom hearts.........
~Hero Kirby's imagination

i just noticed some more things about anti-form...
like donald pisses himself when he sees it
[/lie]