Hi, Jorge!
Since these are older works, I'm not sure if any of my comments here will prove to be useful for you, but I will mention that I see your second image is much stronger than your first, which may possibly be due to any progress you've made with learning anatomy, and whatnot.
Even though the second-image seems to be a quicksketch, it still resonates much better because it's seems as though you put some thought into refining it to an extent, which doesn't seem as evident in the first image, which (in comparison) comes off as rushed.
Your choice of muted-colors there is certainly fine, but they haven't been used to potency, which is to really help establish the general forms, and muscle masses of that creature. With that said, you've definitely done a nice job establishing that this is a no-holds-barred, snaggle-toothed, beast! (With what I'm sure is bad-breath to boot!

)
The matte-painting you've posted here is certainly ominous, but also comes off as uninspired to a degree. You've done great work on the entrance there, and the flying-creatures, but that's not enough to really "sell" the picture on the whole.
What I mean, is that with your great setup, (the colors, the bloodied-rocks/etc.) I'm wanting to see something really gruesome going on here, like some shipwrecks, or a trail of dead-bodies, leading towards the entrance, where maybe in the darkness and shadows of that entrance, you can
just start to make out the faint, glowing eyes of some fearsome sea-monster!
