The animation is jagged, but people already said it. Let's go to suggestions: You're moving the head up to open the mouth, but this is totally wrong. Move the jaw, and try to give it a rotation movement. You're moving each piece as a rigid block. Look at its belly. It should stretch, non only move. The same for arms and muscles. I know that moving block is easier because you don't have to redraw all the shades and textures, but the result is differet. Also, use little movements, as //alaas said. Try to give him a smooth movement, and then add the freaky shakes. I hope you don't give up!

He kind of reminds me of Fallout's Super Mutants [http://www.sghi.info/GL/img/Post.Nuclear.PC.Games/Fallout.3_concept.art/8-05.16_Super.Mutants.1_concept.art.jpg].
Anyways, The pixel art isn't bad, I like it, but the animation seems pretty jumpy, and pointless.
I know it's your first animation, but I must say, it's not good. The animation ruins the piece imho.
Still, nice try, my first animation is crap compared to this.

i know what you're aiming to show with the animation, but in the end it doesnt add much to the piece. rather than giving an emphasis to the "freaked out" attitude of the character, it ends messing the complete structure and concept. moving segments without any kind of flow looks unnatural and ends ruining a good idea.

Does look more like a mutated  freak that got some strange colorshifting
virus injected... imo But it sure does look bad-ass. I like it.

Try animating the raising and lowering of his breathing chest first. move/ animate each element a little smoother (max. 1-2 px/ frame).

Some good shading and line art, though his feet seem a little small. The animation is bad but since its your first animation. It just seems very jagged and sudden. You're just moving body pieces around. Try look at this piece http://pixeljoint.com/pixelart/5574.htm. Nice work overall though

yes, but until the white background is transparified, it will NOT go into the gallery. There's plenty of freeware, and apps on the internet to transparify saved pieces like this...

The problem is not do it, but I would not have it ... Thanks for the comments ^ ^

I don't know why I like it... cause he looks weird.

Thats some nice stuff! However, can you remove the white background? It serves no purpose.