I don't have a SHS to measure the head, but guessing that the length from mouth to crown of head at about 2 inches, you'll be wanting a horn about 3 or 3.5 inches long, and slender.
Would wire work for that, starting with a central thick one and wrapping a thinner one around it to widen the base slightly - then a coating of apoxysculpt or other medium? Never sculpted a horn before so I'm just imagining how I'd do it ~
or something like this? (found this on a body piercing supply store!)
Steel Threaded Unicorn Horn- Buy Jewellery
it's actually called the "unicorn horn" - thought that was neat! ^_^ Maybe too fat for TLU - but still cool to know about for future horned pony projects.