I'm not sure how to reply to specific messages but I'm replying to Pete - I think ex-army horses found their way into many new roles.
I think Joe Skinner's Dolly was an ex-army mule (I'd have to check the book but I'm pretty sure).
As to why they'd be bred specially, maybe they weren't but many types and breeds were bred for specific roles, such as mining, pulling a carriage rather than a cart - as you say above, they needed to be small but strong so anyone who had a good horse, might decide to breed the next generation.
It's just a thought at this stage - Darwin uses the breeding of horses for specific roles as an example of human intervention in the evolutionary process so it was well-established by then.
But that's the nature of research - I have to pin down evidence, rather than assume that things were a certain way because that seems most likely to me.
I was told by someone who I consider an expert that horses made an easy transition from the cart to the narrowboat - but I've also come across an interview from a horse-boater that said ex-cart horses didn't make good towing horses at all, because the boat didn't respond to the pull in the same way. Once I've found out as much as I can, I look at the balance to form any sort of conclusion (which is often still 'we can't be sure'!).