I am so very, very sorry! I'm sending you a lot of hugs, but you'll have to send some right back to me, as I have tears streaming down my face. What a beautiful little thing she might have been, but even in her deformity I see only beauty anyway.
Please don't give up on your mare yet! Do people who have a Downs child always have a second one in a row? No!
We have close friends with twin sons, and ONE of the twins is Downs, and the other was valedictorian of his senior class and in college right now. genetics are genetics, whether equine or human, and although I know how extremely difficult this must be for you after all the months of waiting and waiting, and seeing your mare so sad, I think I would try it again, but with another stallion.