I'm not very good with pixel art, but my friend does a lot of it, so I have the general idea of what to do, and how to do it. I just have a horrible sense of placement. xD I also critique hard, sorry if it's any offense to you. ;A;" Anyway.
First creature: You have an awesome imagination, and that's a good thing for art. However, you need to play with the hues of the colors, and not stay with one color and just change the shades. When zooming into a pixel, the contrasting of the shades, highlights, and base color will seem right. But when zoomed out, it will look blended. Don't be afraid to darken or lighten the colors too much, because it will look right later on. The tail is good, although you need the tail to have more motion. It doesn't stay in an arc like that, but falls downwards, and only has a little bump at the top. As for the spikes, you might want to make them look like they're connected with the body.
Horse: Like others said, you need to work on the anatomy, and once you do that, you should be fine. c: Just need to experiment with colors and shading more. Same with the tail, as I said with the first creature. The mane also goes on the head, not just the neck. xD References are always helpful, and they're your best friends.
Mouse: Same thing with the reference. Pixel art is not for gradient shading. It's a very contrasting shading, and if you want it to blend, use techniques like dithering. Otherwise, gradients will look like you blurred the colors in. Look through lots and lots of pixel arts and look at how their colors are.
Dragon: I think you need to work on the outline, and how to make smooth pixel lines. Don't use gradient shading too much, because it's a bit too subtle, and like I said with the mouse, it looks like you blurred the shadows in.
Uncolored Outline: You're getting much better at anatomy, although you need to stop using straight lines too much. xD It makes the picture look more... /searches for a word/ unrealistic then it's supposed to be. There's a section in the middle of the tail that looks a bit un-normal because of that. Same with the front legs.
Other than all of that, keep practicing. c: Look at a bunch of references and sprite tutorials. They're really helpful, and if you practice the right way, you'll make awesome art.