I really appreaciate everyone points I hope my new rattlesnake shows improvement. I added a tongue, simplfy the eye, flatten the head, improve the tale and I'm only using 8 colors. I try to make the pallete look more like a rattlesnake and to make the twist more readable and hopefully more natural. thanks again for the comments.
Pretty solid first piece, welcome!
The palette is likely the point which could improve the most. If you arrange it by hues, you'll notice it has two mostly independant color ramps, with little hue-shifting within them, and not much of a saturation range either.
It could use a purple, and/or a near neutral, at least one more saturated green and yellow.
I'd recommend trying to bring down the count to 8, you should find the constraint will force you into creative color choices =)
There are a few sore instances of banding in the lower half: a black/brown/green 'staircase', and the long strip on top of the tail loop.
I'd avoid dithering in small sprites: although you did it in a very controlled and limited way, I don't think it adds anything in readability.
The tail tip is a bit messy compared to the rest, this is where you need to break realism (in this case the size) if you want to convey the idea.
Example for these 2 points.
Also it lacks a snake tongue!
Well, for someone new to this, I'm impressed 
Hey you got some nice techniques there like selout and dithering: although the top dither is not working as well as the bottom one because your underlying color there does not extend past the dither and does not blend into the next ligher shade in turn. You could probably also use dither to hint at scales throughout the body.
The tail says it is a rattlesnake but the head shape, color and skin pattern doesnt match. It looks more like a tree snake so it may work better without the rattle. Its also kind of short for its thickness and the twist of its body looks a bit unnatural.
The combination of greens and browns on the body is kind of strange too. If you google green snake and go to images you can see some more natural color combinations.
You may also want to leave at least a pixel of padding on your edges when uploading because some browsers can blur and truncate your edge if the color goes all the way to it.
Thanks for sharing.
The two are very different now :)