I recently talked with a man who had been involved in an effort a few years ago to develop a miniature Clydesdale horse. Enough of them were created to build a hitch that was displayed I believe in the Florida area. They felt the effort was successful in that they created in the first generation horses that were fairly consistent in size and smaller with all the traits of the Clydesdale breed.
Update January 2014 I found a website to share:
I was told that the program was set up with using 18 hand tall Clydesdale mares who were artificially impregnated with semen from a miniature horse stallion 28″ tall. The result was horses that matured around 50″ tall. This matches the mathematical formula you would use, 28″ plus the height of the mare (18×4=72) 72+28=100; 100 inches divided by 2 equals 50″!
Now I was actually surprised because I would have guessed that the get would be taller with the influence of the mare’s great size. The babies were so small that they would not have survived without some help getting nutrition until they had grown enough to easily reach their momma’s udder. But an interesting effort I thought I would share. Here is a picture of one of the teams as adults: