for starters if your doing 3000x3000 and then resizing that's why

I suggest just double the HEE dimensions from now on

example:

horse avatars

1000x --> 500x

stable avatars [text]

300x

stable avatars

600x --> 300x

etc

The art looks great so far with the smudge i agree with Sunset!

Also maybe when doing the mane and tail try that it doesnt look so strait or clean cut. No mane or tail is ever strait you always have a few wild going hairs going somewhere else and always you have a curl somewhere.

Also the shaddow of the horse on the ground could be a little more seen, like on the first art piece due to the background and the horse lighting it looks like the sun comes trough the trees from the upper front, but the horse shaddow shows to the side towards us (looking wise) and the shaddow is very very fainted away its very hard to see.

But it looks great so far, your horse itself sure looks better then my art lol well done!