Stretch in-betweens are one of several things that was under experimentation in the Chuck Jones unit at Warner Bros. before 1945. “The Dover Boys” (1942) has the best-known examples but they crop up in other cartoons. You can spot them in three scenes in “Super Rabbit” (1943), twice very quickly and then near the end of the cartoon in the cheerleading scene. Here are some consecutive frames as Bugs moves from one side of the screen to the other.
Ken Harris gets the animation credit on this cartoon, and he used stretch in-betweens a number of years later in “No Barking,” but it’s felt by some that Bobe Cannon was exclusively drawing them in the Jones unit at this period.