I was thinking the same thing but never went so far as to look for one for conversion. I love seeing them in old movies or reading how they transported horse "back in the day". Here, I'd worry about reliability and we have small roads with nowhere to go and finding a mechanic could be a nightmare... but I like to dream cuz it would be cool.I know, so true! It seems like in AUS and the UK, it's more common to transport horses in a box van, rather than a tow-behind trailer (aka float) that is most often used here in the USA. A one-piece horse box truck (aka lorry) would be so much safer, it can't sway going downhill or flip on crazy roads among heavy highway traffic in bad weather conditions.
Back when I was shopping for my trailer, I was considering selling my pickup truck and buying a commercial box truck to convert for transporting horses, and buying a small electric car for local commuting to and from work. But no box truck I looked at was tall enough for horses at all, even leaving out the expense to convert it.