I believe the colors you have chosen for some of your sprite are way too dark.  You can't see the very grayest color since the background overpower that color.  I can barely see it on my monitor.  I think the sprite would look much better with a gray background if you want to use darker colors, otherwise people playing this game cannot see the sprite the way it made.  Defender of the Oasis did this for their RPG for the Game Gear. 



You can see the darker colors in this game since it using a neutral background.  It also there to reduce the screen cross talk issue I believe, but it made the enemies in this game look great.  I think you should try to use a neutral background if you're going to use very dark colors for your enemies.  It'll make your enemies more visible. It's hard to see the three headed dragon's and octopus's body due to the black colors and background. 

As for the dithering issue, it's great if you're working with low amount of colors or putting it on rough surface. I'm a fan of dithering.  I really love the look of the green slimy alien monster and dithering I believe helps.  The lime green gremlin looking like, I think the dithering is unnecessary, unless it has scale or bumpy body. 

Overall, I love the design of the monsters.  I think the monster would benefit more with a neutral color background to make them pop-up better rather than hiding in to the shadow.  It's really difficult to see the 3 headed dragon body's when he has very dark colors put into his body.  If you insist on using a black background, I suggest making the colors brighter, then for the monster's darkest color, which is black, into colored darkest color, like color value, 8,8,8 or 8,8,0.  This is my suggestion.  Keep up the good work!