I will be making a dragon cake like the one pictured below. The body (from cake board to the bottom of the neck) will be 6" tall.

Should I do anything special to stabilize the body. Is it one tier? Two tiers with cake board in between? Or one tier with special dowels?

It's a bit hard to say for sure, but I would think it is a 2 layer cake with a 1/2 ball on top of that. If so, I would put the 1/2 ball on a cake board w/support in the 2 layers.

The dragons body will be 6 inches tall. Do I have to make it 2 tiers, or one tier?

If 1 tier, what supports do I need?

As I described above the finished body would be 6" (+ or -) tall.
A *tier* is one 2-layer cake (4" tall) so you would use one tier. For supports I suggest 3 or 4 plastic drink straws as a 1/2 ball will not have much weight - even covered in fondant - unless you are rolling it extra thick.