Hey, I really like the NES-y fever dream stuff you do, these are proper old-school mutant-robot-monsters.
To get more out of your lighting I'd try separating everything into flat light or shade, thinking solid comicbook shadows. The white I'd promote exclusively to the really important bits and select highlights. Don't be shy about leaving large areas of form in the shadows unmodeled; it'll actually read as
more dimensional with less information due to clearer signals. I also removed outlines and the blobby pattern for simplicities sake, you might want to reintroduce some for consistency with other graphics but watch out for single pixel line mid-tones dividing blocks of colour: this fringing is a type of banding that highlights the pixel grid unfavourably (look at the border of the sclera in your original).
I liked the idea of it moving
caterpillar-like on those tentacles, the antenna is for extra robo-cool. I missed a trick not adding a nice tight specular or two on the tentacles; give it a go on my behalf!
Trying to clarify the sphere-shading thing:
Disregarding any perspective distortion here for simplicity: the shading of a sphere is actually only going to be a true circle if the light-source is directly aligned with the viewer. Given a single point source, half of the sphere (a dome) is lit at one time; if the light-source is aligned then this lit dome matches the dome of visible surface area; if the light were perpendicular then the lit half would divide the sphere at the centre in a straight line; all the other positions of the terminator are midpoints between the circle and the straight: ellipses. You can picture it accurately by imagining the cross-section through the sphere. Any gradation of tone runs parallel to the terminator so they mirror that ellipse, only smaller.
Edit: I done .gif'd the diagram. Hope that's clearer. Maybe I should have just labelled the first two as 'top' and 'side' out of context though.