chapotraphouse

Most fish and indeed most vertebraes on planet earth are Bristlemouths. With numbers in the Quadrillions, they easily outnumber mammals, birds (including owls) and all the others put together. If an alien race just made a quick stop and beamed up one specimen of a vertebrae species, it would likely look like this:

These are the main inhabitants of earth as far as complex life goes. You just never see them, because they are smart enough to stay below 300 meters deep. Just chilling and signaling with their bioluminescent spots. Even when their main food source migrates to the surface daily, they stay down there. In fact, as they get older and their sex changes from male to female(oh yeah, that's a thing), their swim bladder gradually fills with lipids causing them to slowly shift downwards to deeper depths of over 5000 meters (16400 ft). They seem to hear the call of the abyss. What do they know, that we don't?