I had a lot of success with having the horse follow spooky vehicles. Enlist a friend to drive the ATV (or lawnmower or bike or golfcart, etc...), and just follow on your horse. If he's terribly nervous and more confident starting on the ground, start there, but you can easily do this from the saddle.
Following the spooky object gets the horse's confidence up because he's doing the chasing, not that horse-eatin' ATV. As he gains confidence, you can move him up to walking, trotting, and loping alongside the vehicle, and eventually allowing the vehicle to follow him as he learns that it poses no threat. The same method can be used to desensitize to hand-held things (like chainsaws, for example) - just carry/wave the thing and walk away from the horse, either "pulling" the horse behind or having a friend lead him after you. Some horses get rather excited about "chasing" the spooky object, and will become playful and eager to continue.
Ultimately, the best thing to de-spook a horse to trail monsters is to just get out there and experience them. It's hard to replicate things like deer in the arena to prepare for that. My recommendation is to start riding with a friend on a seasoned and reliable trail horse - having a calm and relaxed buddy can do wonders for a nervous greeny on the trail.
Good luck, and enjoy your trail riding!
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown