When you think about it, spooking in an arena and not on the trails makes sense. If the horse is used to having "unlimited" room to run away if something scary pops up, it will naturally be very timid in an arena where he or she knows that there's less room to move away if something comes out and bites him. By staying away from scary objects completely, your horse is saying "I don't have enough room to bolt if necessary - why should I antagonize (come closer to) something that might eat me?" Obviously, your horse trusts you enough to be good on the trails, but takes confidence in the fact that, need be, there's a place of safety -space - available if need arises. You should work with your horse in confined spaces. Trailering, roundpens, stalls, and arenas are all places that provide your horse limited space. Try working from the ground with your horse in all these types of places, and improve his or her confidence in small spaces as well as incorporate some sort of desensitization program explained above. You'll gain your horses confidence in tight spaces in no time!