The Horse Head is not in the Great Orion Nebula. (M42) The great nebula is the middle of the sword. The horse head is near the leftmost star of the belt.

The horse head is very hard to see. The smallest telescope I've seen it with is a 25". From a good dark site. I had to use a Hydrogen-beta filter to see the red nebulosity the horse head is in front of. It still took two hours to be sure I had found it the first time.