9. Vonderful Muscles
While the origin and configuration of the Klingon Bird-of-Prey can be easily traced back to the Romulan Bird-of-Prey from TOS' "The Balance of Terror", the ship's designers didn't rely solely on Wah Chang's original design.
A novel approach for Star Trek features at the time, The Search for Spock's production bypassed the traditional route of using an art department to develop concept artwork which was then handed off to the VFX house; director Leonard Nimoy instead allowing Industrial Light & Magic to develop starships and environments all on their own.
During early discussions with Nimoy, ILM's Nilo Rodis, David Carson, and Bill George were inspired by the director's imitation of a predatory bird, arms outstretched like wings. Nimoy's guidance also included the directive that the ship should possess an elongated neck, again a characteristic of an attacking bird and one that would be incorporated in Klingon starship designs for decades to come.
Further exploring various concepts for the ship, Nilo Rodis created a drawing of a muscle man, arms flexing in a downward position. Like Nimoy's literal interpretation of "bird-of-prey", this sketch proved the lynchpin in the Klingon vessel's design. Model maker Bill George using the muscle man drawing to create the Bird-of-Prey's distinctive wings-down-attack configuration, incorporating large vents on the top of the ship to mimic shoulders, and even including piping along the front "head" of the ship to mimic the look of a muscular football player's chin guard.
Far from the alien vessel it has come to embody, the Klingon Bird-of-Prey was inspired by Leonard Nimoy pretending to be a bird and a vaguely football player-esque drawing of a muscle man.