Like most people I assume it is not around anymore but a Cassowary is (probably) a kosher bird which has one one horn and many colors (including the skin under the feathers)
Of course I don’t think it is the Tachash. I’m mentioning it as an animal that 99.99 percent of people never would had heard of that meets some of the criteria being discussed (kosher, colorful, one horned)
Another such animal would be a Saola which a one horned deer whose current natural habitat in Vietnam and Laos . I don’t think it is the Tachash either (although it is more likely to be it that a bird)
You never know what others are out there