When creating large bosses in the NES artist and programmers must find ways around many different limitations. Scanline limit and on screen sprite limit makes using sprites difficult so many games created bosses from the background layer. Animating the background boss was tricky though because only 32 tiles could be changed at a time.
One way large animations can be achieved is through blending frames.
For example, take this large background toad opening its mouth for an attack. Each transition is way over the 32 tile change limit so what we will be doing is blending the frames in between using bits of the last frame to reduce the amount of new tiles needed. This allows us to have a large, animated boss, without breaking the tile limit restrictions.
Below we can see the blending of two frames by using tiles from each frame.
To illustrate, here is the tile count per frame.
Here is a gif of the final animation, played at the proper speed. Now we have a large, animated NES boss, which is fully within the limitations of the NES hardware. It is important to mention that the end animation should be played at a fairly fast pace so it looks smoother.
For reference and education purposes, this animation has been slowed so that the tile blending technique for NES bosses can be seen in real time.
And that's it! The graphical limitations of the NES and other retro systems can be a chore to deal with, but they can also squeeze creativity out of you that you didn't know you had. Using blended frames for NES bosses and boss animations is just one technique that can be used to push past the restrictions of these retro video game systems.