On the subject of alien-seeming animals inhabiting our own planet, consider the giant larvacean, undersea "gelatinous invertebrates" that look like "ghostly tadpoles" about the size of breakfast muffins:Snot Palaces
Although the creature's photo brings to mind a jellyfish, it belongs to a different subphylum, the Tunicata. As far as I can gather from Wikipedia, they're mostly hermaphrodites, an efficient type of reproductive biology because any two members of the same species who happen to meet can produce offspring together. With bodies made mainly of water, larvaceans are filter feeders. They construct "houses" around their bodies by secreting mucus in the desired shape and blowing it up like a balloon. These delicate "snot palaces" (a nickname that belies their fragile beauty) have an inner and an outer house. The outer layer provides protection and filters water. The inner structure collects the food from the water. Since the creature's house gets clogged up quickly, it has to build a new one every couple of days. Despite the small size of the larvacean itself, its house can measure at large as one meter.
Would a larvacean's house be counted as part of its body, even though it gets frequently discarded and replaced? Animals such as snakes, after all, regularly molt their skins. The larvacean's "snot palace" isn't alive, but neither are mollusks' shells or our hair and fingernails, all of which tend to be thought of as body parts.
Could a creature composed mostly of water have a brain or a structure that serves as one? Hmm—neurons communicate by electricity, and water conducts electricity, so why couldn't a gelatinous invertebrate have an electrical network that performs brain functions? And couldn't any brain, if its environment demands higher functions for survival, develop sentience? Suppose a larvacean-like animal extruded a mucus "house" with a structure complex enough to support an intelligent electrical network? Of course, every time it replaced that structure, it would have to transfer over stored memories and skills. . . but if we can imagine a cloud of energy with sentience (as in more than one STAR TREK episode), we can imagine a mind built from water and mucus.
Margaret L. CarterCarter's Crypt