The zodiac's name means "Serpent Bearer", so based on the Wikipedia's information, the zodiac is the concept of balance, like Ying and Yang.
As the snake continues to wrap and entangle itself around the man, the man continues to untangle the knots and loops of the snake, in an infinite cycle of power.
The way I see it:
You could either make a heartless man, like Darkside, with a snake around his body, which probably would be hell hard to make.
Or use one of the 3 creatures below to create some kind of two-headed heartless.
Of course, it's all up to you what and how this thing looks like.
images1.wikia.nocookie.net/__c… Duon from Super Smash Bros. Brawl
images2.wikia.nocookie.net/__c… Thresholder from Kingdom Hearts 2
images1.wikia.nocookie.net/__c… Lost Number from Final Fantasy VII
That'd make it too easy of a target, since the heart would be the weak spot.
Remember, this zodiac is like the symbol of infinity, two bodies chasing each other in an endless cycle. (Pisces does that)
Of course, you've got to remember that this zodiac was removed as time went on, so It's not officially part of the Zodiac anymore, just something that was there for the moment.