Well let's see if I can say anything you wouldn't know.
I'd say the colors on the last dragon recolor are really poorly chosen. White outlines are generally a bad idea for a normal sprite, it's very irritating on the eyes.
The wing anatomy is pretty unappealing on all of the winged creatures on this page, they are unresearched and pretty clichely repeat the solutions of other people. For example, the "elbow-finger" on the baby dragon and the demons is really common on wing designs done by young artists on deviantart.
On the adults the wing shape is unclear and hard to understand how the wing is folded (also the fingers are folding in a curved arc that further makes it more of a symbol rather than any real shape.
The green and red slimes lack interesting color variation, the yellow slime is the most successful in that regard.
The bigger tree with the wasp nest is flat and the nest shape detail is too spread out. The smaller nest does it much better.
The rest of my commentary is just about personal preference, but I prefer designs that are less spindly. For example the wraith, scorpion and flying snake are all rather thin and don't show off much detail. For the snake it probably would be fun to have a bigger head.
Play with contrast on the size of your creatures - small body + big head, or medium body + big limbs + small head or something like that.
On most of these, they're all kind of medium body + small head etc.
Hope there was something new among this stuff.
Edit: I was bored so I tried designing some of the monsters to be more interesting and maybe better for pixeling. (I haven't tested them in the arena of pixel art tho.)