Start with a funny-sounding word. This is the start of your story.
We know little about this creature, thought to be a precursor to some of the less well-understood flightless mammals that today inhabit remote and densely wooded islands in parts of the world that turn out to be surprisingly difficult to get to. Fossil records suggest that the Illusory evolved around the time of the Archaeopteryx, although specimens are less conclusive, and curiously apt to go missing from museums in which they are being studied. The Illusory appears to have been smaller, lighter, and more readily overlooked than more illustrious avians.
Unlike the Archaeopteryx, believed to be the ancestor of all modern birds, the Illusory seems to have lost its capacity for flight, developing instead an extremely refined ability to sidle slowly out of view until the observer has… is… um. Thought to be a direct descendant, the Hooweasel, native to eastern-ish parts of – a place – is reported to induce intense bouts of confusion and… and forgetfulness? Of something. When under attack.