After careful examination of several practically-identical products and a thorough delve into reader reviews, I can finally make my long-awaited recommendation for a strap-on unicorn horn: the YanJie Shiny (~$10, Amazon).
Aside from the critical rainbow model, it comes in various colors, is adjustable, and has the all-important strap so it doesn't come off your head when under extended or vigorous use.
It is five inches long, made of polyester, and a buck or two more expensive than the competition. For an item so important, though, it's worth splurging.
"I am an 'average sized' adult and I wore this for a party," writes D.T. "it fit perfectly."
"Using for cosplay and cosplay only," reports Queen of the Succubi. "It is legit enough."
"We love it, dog hates it," cautions Daniel. "What more can I say."
If you know of a superior shiny rainbow strap-on unicorn horn, tell us about it in the comments!
YanJie Shiny Unicorn Horn [Amazon]