Singular word that ends in 's' seems to be rare, not counting proper nouns. So far I only know lens. Is there other example?
If you allow multiples of the letter s, there are a ton (class, pass, mass, etc).
If you restrict it to words that end with a single s, I suspect there are still a great many. Dias, bias, alias, octopus, cactus, and mucus all come to mind. Come up with common word endings that have an s and I think you can generate more.
In keeping with your example of "lens," add "iris." In keeping with the puzzle nature of the question, "rebus."
Any medical condition ending in -itis, e.g., "bronchitis."
Any word ending with the suffix -osis, e.g., "hypnosis."