First thing to remeber horses do things like that when they are bored...make sure your horse has plenty of grass or hay to occupy his time...if this don;t work you can try a grazing muzzle this would prevent him from chewing things..but will still allow him to graze and eat and drink water.
Just to add to Betty's advice about your Standardbred chewing. Get his teeth checked by a horse dentist - if he's never done it before then his teeth could be causing him problems. Another remedy to use to stop him from chewing up fence posts if you didn't want to use a grazing bucket is to paint Stockholm tar on the rails and posts. It's messy and horrible but it works a treat. Best of luck.