The "Beerhead Baetis" is a popular trout fly pattern, specifically for imitating small mayfly nymphs (Baetis/BWO), known for its effectiveness, especially in tailwaters, featuring a beadhead, olive/dark body, and often "legs" made from ostrich or other fibers for movement, with variations like Kyle's Beerhead using root beer-colored resin for attraction. It's designed to mimic aquatic insects during various life stages, providing trout with a natural, yet enhanced, meal.