My horse is foaming at the mouth, what could be causing this?

When examining a horse that is foaming at the mouth, there are several possibilities that come to mind. Oral problems are at the top of the list. Most often the problem is a fractured tooth or some foreign material (usually grass awns or twigs) embedded in the gums or tongue, or feed material packed into a pocket between two teeth that has rotted and caused infection. A thorough oral examination with a speculum and good light source should reveal the problem.

Other causes of foaming at the mouth include choke, but this is usually accompanied by retching, feed material coming out of the nostrils, and signs of distress. Most cases of choke are mild and resolve fairly quickly on their own, but stubborn situations can be treated by your veterinarian with sedation, anti-inflammatories, and gentle lavage with a stomach tube.
An uncommon cause for an "outbreak" of drooling in a barn is a certain plant containing a toxin called slaframine that is found on pasture and in hay containing clover. The condition, accurately named "slobbers", clears up when the offending forage is removed with no lasting effects. Some horses that are receiving oral medications will get very pouty and hold the drug in their mouths for a long period of time rather than swallowing it. These animals will appear to foam because they are not swallowing the copious amounts of saliva that is naturally produced. Foals with gastric ulcers will occasionally drool as well.

Finally, one must consider disorders of the pharynx including abscessation and neurologic dysfunction, as well as certain diseases such as rabies and encephalitis.
If thorough rinsing of the mouth and a (careful!) finger check around the cheeks and lips doesn't reveal the problem, give your vet a call.