Let's face it: A lot of Star Trek aliens over the years, on The Original Series and later ones, were no more than actors wearing a weird headpiece or a few bumps on their noses. A lot of this came out of creator Gene Roddenberry's invocation of the "parallel worlds" concept, meaning that there were theoretically millions of planets in the galaxy where humanoid life had evolved very closely to that of Earth and even shared some similar historical developments.
Of course, this was mainly a budgetary thing: They only had the money for, say, a frontier town on a planet that resembled the Old West and whose inhabitants had a rather large ridge across their forehead (I'm making that up, but some episodes were not that far off!). But The Original Series, even in those days of little money and relatively crude special effects, managed to pull off a few life forms that were truly alien -- and in some cases, almost beyond the understanding of the crew of the U.S.S. Enterprise.
Here, then, are 15 of the strangest, most exotic and most, er, alien aliens Star Trek ever had trundle or shimmer or coagulate across the screen. I'm not saying that the show pulled all of them off effectively, but they were all at least one hell of a lot odder than a guy wearing just a pair of pointed ears ... oh wait.