You’ve got some really cool stuff, I would say the biggest thing that is lacking from what I see is real life reference. So for example, the guy who looks like he is wearing bicorn hat, putting eyeballs in and sculpting around them has given it a lot more form than what the first or last creature has, though he has no lower eyelids. The main thing I have learnt with organic sculpting, especially for non existent beings, is getting an idea of the bone structure underneath, the muscles laying over that and then the fat on top of that is really important. You’ll find a lot of anatomy remains the same no matter the shape or size, so for example you can compare the anatomy of a quadruped dog with that of a horse, the front legs for example have the same muscle groups in the same places, basically they work in the same way. So with the last guy, his neck would have the same anatomy to a human. With him being so slouched you would also imagine that the top of the trapezius would put a lot of work in to keep his head upright, so it would be larger than in a human but its still comparable. Then if his skin was thick like a lizard or something, getting reference of the loose nature from a komodo dragon or something would go pretty well.
If you plan on learning organic sculpting with non existent beings, I would check out some of Terryl Whitlatch’s interviews, she’s a really cool artist and her knowledge of anatomy and making creatures up to look like they should work is crazy. I would however recommend giving humans a shot, its really hard to get right and people will be able to give you a lot more help and advice. A lot of people upload nude reference up on deviant art, I found going there and getting reference and recreating it from a dynameshed sphere challenging but it took my sculpting and knowledge of anatomy on leaps and bounds!
Sorry for the ramble but I hope its useful!