It amazing what animals can achieve when driven by hormones but its not a cross I'd want to deliberately do even using AI
This is a UK advert for a Shetland x gypsy cob NOTE - The advert is not open for critique or comment
I think it is possible... but like someone said, it's probably not something you want to do. I once knew of an accidental Shetland/Percheron cross from someone in our 4-H club. Not sure how the horses managed that (the sire was an escaped Shetland), but it can happen I suppose. It would be really variable what you would get as a result though.