Rain World has such cool worldbuilding, it's such a shame the mechanics always kick my ass. I agree with @Space-ageWrangler about the tentacles, they seem odd so short and with the creature unattached to anything due to their shortness. You did a great job on the shading though, the slugcat has such weight to it.

Rain World fucking rocks.

On a side note, I think the lighting is pretty good (especially on the main body) but the tentacles could probably use some work. I think they're too short compared to the size of the long legs, maybe it would help to join a few down to the ground like in the game, that way you can get a feel for the size the rest of them should have.

When I have to do something like this I also like making them curve around in big arcs and wind around eachother a lot, so that can help you add detail too.

Also, maybe you could have used different arrangements for the bubbles? It sorta sticks out that it's always the same pattern.

Edit: I think the background's fine mostly, the gradient gives a nice underwater effect and the rocks on the bottom are fine, it's just the bubble thing that bugs me.

Pretty cool