Hoof Pick: This is SO important to have with you. If your horse picks up a rock you'll need to be able to get it out of his hoof before it causes him to go lame. A pocket knife can be used but there is a chance of harming your horse. Make it a habit to bring a hoof pick with you.
Hoof Boot: If you're going on a long ride and your horse has shoes, a hoof boot can be a real lifesaver. There is nothing worse than a horse throwing a shoe 4hrs away from the trailer. Even if your horse is barefoot, a hoof boot can still be helpful if he gets sore on rocks or rough terrain. Hoof boots are kind of pricy and more of a luxury to have. If you have the funds or already have them, it doesn't hurt to bring one along if you have room in your bags.
Cellphone: A phone is a must have in case of emergency. Rather then being in your saddlebag, try keeping your phone on your body. After all, if you get thrown off your horse and are hurt, your only method of communication isn't going to help if it's miles away on your horse.
Emergency Vet Phone Number: If you don't know the number off hand, write it on a piece of paper or enter it on your phone. If your horse is injured while on the trail this number will be valuable. They can come out to you or you can give them a warning that your horse is injured and you are trailering it in.
Bandana: A bandana is such an amazing, versatile tool for trail riders. You can wear it yourself to tie your hair out of your face or to help prevent breathing in dust. Bandanas can be used for first-aid purposes to stop bleeding or clean a wound. In an emergency they can double as a lead rope.
Sharp Pocket Knife: It's a good idea to keep a sharp pocket knife on your belt or in your saddle bag. A knife can really come in handy if you need to free your horse from a tangle. You can also cut away small twigs or use it to fix broken leather tack pieces.
Baling Twine: Baling twine is extremely useful. In fact, it should be called horseman's duct tape. Baling twine can be used in place of a broken reins or a lead rope in a pinch. You can also use a piece to replace a curb strap or stirrup keeper. A few pieces of bailing twine rolled up with a rubber band to hold it together doesn't take up hardly any space and can be an amazing tool for tack emergencies.
Water Bottle: It's important to keep hydrated, even on short trail rides, especially when it is hot out. Keeping a backup water bottle is a great idea, especially if you won't be going by any clean water sources. Your main water bottle can be in a special water bottle or canteen holder that clips to your saddle for easy access. If you'll be riding for a while and want cool water, consider buying a thermal saddle bag that will help keep your food cold.
Snacks: If you'll be in the saddle for a few hours you might get a little hungry. Keeping a couple snacks like trail mix or granola bars can help settle your stomach and give you something to nibble on. Jerky is another easy to eat snack that is pretty filling. You might even want to bring some carrot sticks or an apple if you'd like to share your snacks with your horse.
Halter & Lead Rope: Always keep a halter and lead rope on your horse during a trail ride of any sort that takes you off your property. We already know the dangers of tying horses by the reins. You never know if you'll have to get off your horse and tie him. Nylon, web or leather halters can be pretty cumbersome or uncomfortable under a bridle so soft rope halters are recommended. If your halter is really affecting the horse's bit, you can keep it in your saddle bag. Don't drape it over your horn(if you ride western)! It is far too easy to get tangled up in it if your horse spooks.
Rain Slicker/Poncho: Emergency rain slickers or ponchos are a good idea to keep in your saddle bag if you'll be out riding for a while and there is a chance of inclement weather. Based on personal experience, it really sucks to get stuck in rainy weather, especially if you're in a leather saddle. These emergency slickers and hooded ponchos pack up very small so you don't need a giant saddle bag to bring on with you.
Gloves: For those who don't already wear gloves while riding, you might want to keep a pair of sturdy leather gloves in your saddlebag. Moving debris, blocking your face from twigs, cleaning out dirty hooves and other such activities can be pretty rough on your hands. Gloves keep your hands clean and help prevents cuts and splinters.
Trail rides are super fun for horses and people. If your saddlebags are stocked with basic items you're sure to be prepared if any unplanned events occur on your ride.
*Image courtesy Flickr creative commons.