If you search the web, you will find very few references to horsehair flies, and some of the ones you'll find will be vague, referencing a fairly nondescript fly with a woven hair body tied by guy in Montana a hundred years ago. That is not what this page is about, not by along shot.
This is what I'm talkin' about:
Horsehair is just slightly translucent, so it gives a horsehair-bodied fly very different visual impression than would a body of any other material. Brown horsehair actually has a striking resemblance to the nymphs of several mayfly species. The exoskeleton of a nymph is also slightly translucent.
A pheasant tail nymph would match the color of the nymphs, but pheasant tail isn't translucent and it isn't nearly as durable as horsehair. A pheasant tail body, even one ribbed with thin wire, can get destroyed by the teeth of the first trout to hit it. A horsehair body will outlast the hackle. You'll probably lose the fly before the body is ruined.
I remember the first time I fished a fly tied with a brown horsehair body, years ago. It was very late in the year, my last trip for the season. At the end of the day, as I had worked my way back to the bridge where I would leave the stream and walk back to the train station, I thought I should at least try the horsehair fly, which I hadn't yet fished.
With little time left before I'd have to get out of my waders, make the trek to the station and catch my train back to the city, I tied on a brown horsehair spider and promptly caught a fish. In the winter, I never promptly catch fish. In fact, in winter, one fish is a good day and two fish is a great day.
I released the fish and promptly caught another! Whoa! That just doesn't happen. Unfortunately, by then I was out of time so I left, vowing to fish that fly again the following year.
Over the course of the winter, some other pattern or patterns caught my fancy and I didn't fish a horsehair fly again for a long time. Why? Who knows? It was a long time ago. Who remembers why he didn't fish a particular fly years and years ago.
Luckily, I didn't forget about the fly entirely, and did fish it a few years later. It still worked!
Unfortunately, you really can't see more than the very head of the fly in the trout's mouth but it is indeed a brown horsehair spider. You can see the fly a bit better in the photo of the bluegill below.