We have a gypsy family (one of many housed here in the village) who are often seen driving their 4x4 pick-up, with a kiddie sitting on a bale in the back and holding on to a coloured cob which is happily trotting steadily behind.
Now, I know it's not exactly the done thing, but these ponies and cobs are completely traffic proof, child & dog friendly and very fit!!! Only thing I really worry about with these people is that they start them off far too young IMO.
They used to lead the stallions down from a 4x4 where I worked locally - but the old guy would sit in the back leading them out the back hatch - he was 70 and couldn't walk the 1 1/2 between the years - and it was a private estate
Personally wouldn't do it, but have seen folk DRIVING and leading horses VERY recently
When I kept my boy in Leeds the field was down a quiet private lane from the stable yard. The guy who I shared the field with would lead his cob out of the drivers side window of his 4x4 with the horse cantering on the verge.
I have to admit I did do this with mine a couple of times but only in walk - out of the drivers side window and TBH my horse took it just fine. In some ways I think it helped him with roadwork as now if he won't pass something scary on the road he will happily follow a car by it as if it was another horse.
However I would never ever consider doing it on a public road.