i don't use gimp haha but mane and tail smudging/drawing
=) If you go to deviant art, look up 'photoshop mane and tail brushes, or horse brushes. You should be able to find brushes that are downloadable, for quick use. That way if you have trouble drawing manes/tails, you can just use a brush instead. =)
Manes and tails are hard as heck to get to look right when you cut/paste them onto a new background, although it helps if the background you are using has quite a lot of depth to it as well, as it helps take away from the edges of the copied object. EXAMPLE:
The easiest way to make them look good is to zoom in to about 500-800% and cut the object you want out, bit by bit, at pixel level so you can have as accurate a mane and tail texture on the edges as you can. And then after the image you want to be placed on a new background is all cut out, select it, and move it to the new background. Make sure that you have a TRANSPARENT layer between your background and the image you are pasting onto the background. Paste the image onto the background and place it where you want it to be, then zoom in to the 500-800% range, and take a rather small brush and blur
the edge of the object on all sides. Use a smaller brush for the areas you may want sharper than others, and you should have a smooth cut out image on a new background. =)
If you want to have part of the background in front of the object you pasted on, before copying the object onto the background, go through the same process as if you were to copy it to a new background, and then just paste it on top(again new TRANSPARENT layer in between), and then move it to where it would've been if it was still on the background(like in the image shown above).