How much can a horse pull?

As a horse can pull so much more than he can carry it makes me wonder why people started riding at all. Maybe I should trade my 2 neds in for a mini shetland. Running costs would be so much less and he could live in the garden! My Mum is going to get a driving pony so I will have a good practice on that to see how much I really like it. Is ok having these romantic ideas but in reality its expensive to get into if I end up only riding her!
 
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