Danny
I have seen the Horsehead on most occasions I have looked for it with my 12.5". However the thing is you won't see it like the pictures. I detect a missing chunk out of the background nebula IC434. My best views of it came from an excellent 22" scope from dark site near the zenith, but even then it was hard work to see any of the detail shown in pictures. These observations have been unaided. I have heard of smaller than 8" scopes detecting it (which is really all you can expect to do anyway) using low enough power to increase the contrast of the background nebula, but high enough to keep the nearby bright stars out of the field. Good luck with the hunting.
p.s. the OIII filter is too agressive and will make the background nebula disappear. A UHC filter lets through some H-beta so it can be used to help, but ultimately a H-beta Filter will provide the best assistance to seeing it - not that I suggest you buy one - not useful on many other objects unfortunately
