We had a treadmill at a racing stable where I used to work. All of the horses seemed to take to it pretty quickly. First time out we would have one person at their head and one person level with their hip with a whip, ready to tap them up if they lagged. The treadmill was started very slowly until the horse got the hang of it and only at walk for the first few sessions. Once the horse was comfortable we would commence trotting and eventually dispense with the header. Can't say we ever had a problem and the horses seemed to actually enjoy it.