The next closest body of water is the neighbor's pond, though due to large property sizes, it is quite a distance! I always see him in the trough, so I just cannot figure out what he's eating.

Horses put their lips just far enough into the water to suck it up through their teeth, but that's an interesting point that he may ingest the backswimmer! And no, in my experience, horses do not spit things out. No other animals drink from the trough.

I am thinking it won't be a problem, because the times I've had to put my hand in the trough to remove bits of hay that have fallen in, the backswimmer always moves away from my hand. I just don't understand why he likes it in there so much that he keeps returning when there's no good food source for him! Since they have the ability to fly as adults, is it possible he's leaving the trough to feed on land, or do they feed exclusively in the water?

Backswimmers will stab an unsuspecting victim if there's accidental contact. I've heard it compared to a wasp sting; it can be painful for a long time. I'd not recommend that you allow it to remain in the trough.

They feed only in the water.

Yes, a backswimmer may not be able to move away from the suction of a horse drinking. Obviously the insect can live for some time without feeding and I speculate that it is far enough along in its lifespan that it has mated and feeding is no longer important, also no energy is being expended in flight.

You were right, The Logician!! Just now, I was out with my horse and decided to watch to see what the backswimmer does when my horse goes to take a drink. He was very close to my horse's mouth as he was drinking, and it happened too quickly for me to do anything, but up he went into his mouth! I couldn't believe I actually saw it happen! I feel bad because after reading Rhizo's comment yesterday, I actually removed the backswimmer from the trough, but while walking with him, he fell on the ground and I felt incredibly bad as he was flopping around, so in a moment of weakness, I returned him to the trough, where he continued happily swimming about. Now that he's been swallowed, I regret it! No amount of backswimming will get him out of this predicament :( Lesson learned!

That backswimmer led an interesting, sacrificial life. And if the horse had drank it up days ago, it may have been assumed that the backswimmer flew off to the neighbor's pond. Perhaps the horse is now capable of swimming backwards.

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WaterSurface-dwelling insects can often be caught with an empty jar and lid: submerge the jar near the insect and the resulting whirlpool will draw the insect in, then apply the lid.