These are the favourite horse for a children's mount. They developed in the rather harsh conditions of the Shetland Isles and as a consequence they are a hardy and stocky breed.
There is also an American Shetland Pony, with two divisions of depending on purity of pedigree. I would imagine that the cross breeding might reduce the strength of these animals, but British Shetland's are probably the strongest off all the miniatures.
A couple of pages on Shetlands;
http://www.ansi.okstate.edu/breeds/h...land/index.htm
http://en.wikipedia.org/wiki/Shetland_pony