I think you're a bit off the mark when you say they are just a 'made up' breed.
The (coloured) horses which went first to America were sourced from some of the oldest established gypsy/traveller families in Britain, who have been breeding these horses for generations, and know pedigrees off by heart.
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Many of the waggon horses are between 13.2 and 14.2 hh, and they manage the job very well.
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Some of the American buyers came over to England to visit the horse fairs with the gypsy breeders who were their 'advisors' on their route into coloured cobs. Studs were visited and the buyers went away with some of the best examples of the types that their big money could buy.
Horses such as 'Galway Warrior' and 'Drummer' (one of the English queen's parade horses) went to America to start stud work there and, being America, the prices for stud fees and young stock rocketed to heights previously only dreamt of.
I don't see why you'd refer to these types as 'made up breeds' when you have such oddities as Tennessee Walking Horses and American Saddle Horses (the conformational differences between which are a mystery to me), not to mention the American Shetland Pony..........all nice in their own right, but no less bred for purpose than some lines of gypsy cobs.