These are really weird aliens. They are usually distinguished by two or more of the following:
• Non-vertebrate or at least radically non-humanlike biology.
• Nonhuman mental behavior and personality
• Either unable to survive in Earth-like conditions, or able to survive nearly anywhere.
• Vastly different motor, sensory, or communication abilities.
• Completely nonhuman reproduction, such as budding, virtual immortality, and a queen (like ants, bees, and termites)
However, they are still like people because
• Some kind of language, not necessarily verbal, we can learn to interpret (Or maybe not, but we can at least recognize it as a language).
• Their own belief systems, however unusual.
• A mind-set that admits to things like logic and intuition; not necessarily those things by our definitions, but things like them.
• At least some resemblance to living things with which we are familiar. They eat, sleep, reproduce, etc.; they are clearly organic beings, or else Mechanical Lifeforms.
However, in many stores they are simply animal-like in intelligence.
Because of the long, strange history of life on Earth in real life it's very likely if we ever actually encounter alien life, it might fit in this category. This is arguably because, species that evolve naturally would have adapted to solve similar basic problems: obtaining food/necessities, negotiating natural disaster, adapting to new circumstances, avoiding contamination by germs, competing with other species, competing with themselves, and so forth. So we would expect to find them at the very least odd.