Hypothetically, why would happen if someone tried to ride a pony, horse or mule that hasn't been ridden in a while? The equine would have been used to farm work (pulling carts, plowing), so it's used to harnesses (or whatever you call that... I'm not a horse person).
The character in question (since this is for a story I'm writing) buys the equine off a farmer because he has to leave quickly. Would it be different for mules, ponies or horses?
The character hasn't ridden in quite some time either. On a scale from 1-10, how much disaster would this be?
On a similar note, what would the effects be for the rider? If he does manage to ride the horse, will he experience back pains or leg pains or butt pains or anything else by the end of the day?
Can't speak to other equines but I came back to my horse after 10 years of not riding her and it went fairly well. She was a tad nervous but no major episodes (I'm a decent rider though, not a beginner, so I don't push her past what we can handle). Horses don't really forget their training, they may be rusty or testy but it's a bit like riding a bike for both parties.
However, a horse being used to farm work and harnesses doesn't mean it's been trained under saddle and can just be tacked up and ridden off into the sunset.
Also, the amount of soreness depends on the physical fitness of the rider. I'm still getting back into things now and I've ridden my horse for up to an hour with absolutely no physical effects the next day (which surprised me). When I was younger and out of shape, I'd feel it, but now that I'm in the military and running/exercising regularly, I don't.
That being said, a several hours long ride would probably make me sore for the next day..