Drone 2 is a very incomplete enemy; the only coding for it are five directions for one frame of animation, and the only sprites for it is a front, side and back view.
Three of those enemies can be seen flying in distance in the still of the title and loading screen, one of them firing. The rocket drone also is featured on this image, confirming that Drone 2 is not an early design for the other drone, but another enemy. Furthermore, the green drone is referred to as "DRONE1" sometimes in its coding, also suggesting that there was supposed to be another Drone at one time.
Since the Drone 2 has no attributes, there’s no way to see what it might’ve eventually become.