One episode of the BBC series PLANET EARTH: BLUE PLANET II highlights denizens of the ocean depths that thrive independently of energy from the sun. They rely on energy from other sources, and some have no need of oxygen.
Some live in methane-rich environments known as "cold seeps" or "cold vents":Cold Seeps
These spots aren't "cold" in the absolute sense, just less hot than the hot vents referenced below. Bacteria, mussels, and tube worms live happily in the methane or hydrogen sulfide of these ecosystems. Some individual tube worms have been estimated to survive for 250 years in such locations. If similar life-forms developed on other planets in environments like these, in the absence of competition from oxygen-dependent and sunlight-dependent creatures, and eventually became intelligent, a lifespan of that length would allow them plenty of time to learn and pass on their learning to future generations.
Other organisms have evolved in the volcanically active areas around hydrothermal vents, where water can reach temperatures of several hundred degrees Fahrenheit:Hydrothermal Vents
Like inhabitants of cold vents, life-forms in hydrothermal vents also depend on chemosynthetic bacteria for food. Crustaceans, tube worms and other types of worms, gastropods such as snails, and even eels are among some of the creatures that populate these locations. It's believed that life on Earth may have originated in an environment like this. Again, on a planet where this kind of environment dominated, we can imagine that hyrdrothermal-vent species might evolve sentience and intelligence.
So living creatures can exist right here on our planet in conditions that would be lethal to most Earth species. The quest for extraterrestrial life needn't confine itself to oxygen-rich environments. Moreover, we don't have to expect advanced beings to conform to the familiar humanoid shape. In Heinlein's HAVE SPACE SUIT, WILL TRAVEL, the teenage narrator describes the villain, an invader from a distant solar system. He's puzzled that these decidedly inhuman-looking aliens can survive in Terran environmental conditions, until he reminds himself that spiders resemble us much less, yet they live in our houses. We don't have to search beyond Earth's ecological systems to find bizarrely alien creatures.
The Wikipedia articles include some color photos of those exotic organisms. Take a look.
Margaret L. CarterCarter's Crypt