I think you can do wonders by switching art styles for the surreal/abstract creatures. For example, one of the monsters may look like a Picasso painting, another one may look like a buch of photos stuck together, etc....
Look at abstract art for reference. This can help when you feel blocked.
Think of the symbolism behind the creature. This will give an idea of it`s general appearance.
Another tip is to not be afraid of mixing things up. Write random words on some papers and pick three of them at random. Then combine these into a creature. You will probably get something freaky.