It depends if this is BW or B2W2. If this is BW, I would recommend Haxous as a good physical Dragon type, but if you already have too many physical based Pokemon on your team, go for Hydreigon. It has STAB Dark as well which will be useful against Shantal and Caitlin.
If this is B2W2, I would not recommend any Dragon type, simply because you need to be over trained quite a bit if you want to use your Dragon against Iris and not have to heal between every move. Since Dragon is x2 against itself, it's a double edged sword to use a Dragon type against her.