I am thinking about doings 3-d dinosaur on side of cake and am wondering what the best sub structure for the rkt? I have never done anything like it so all advice on the aspect of 3-d structures is welcome.

I not sure what you mean by sub structure? Are you trying to do like a dinosaur cut out for the side of a cake or a dinosaur as the cake?

Yes, like a dinosaur standing on the side, clawing, maybe eating the cake. Really I'm curious about any type of 3d figure molded in rkt and wrapped in fondant. Do most people use dowels, for ex like a dinosaur in the legs, forsupport? I worried about a figure being too heavy and the rkt collapsing. I saw a cake pic here where Dowls were attached to a base... Not sure though. How long in advance would one make such a figure?

With RKT you can make it way in advance. If it is going to be BIG - I would put support in. My theory is; if you are worried about it being to heavy and needing support, then you probably need support. You can use dowels for the legs, then use plywood on top of the dowels and build your dinosaur on top of that. You can wrap the legs/dowels with modeling chocolate or RKT, then crumb coat and fondant. Someone else way have a better idea.