I’m playing around with models at the moment - I’m no great artist, but I don’t completely suck either- if I get anything good together I’ll be more than happy to share.
I don’t know if anyone else has done much of this, but the easiet way I’ve found to create a beast if you’re thinking of doing your own, is to create the model in 3D - as in actually in real 3D using Plasticine to get the overall shape, then photograph it from different elevations and line them up in the modelling program (this may be quite tied to Milkshape, which is the one I use because it is cheap, simple and intuitive) to match points and vertices. A bit laborious, but easier than working from sketches and requires less imagination as to what the finished thing will look like.
The other thing to investigate for natural features is using fractals to generate them automatically - iirc realistic (and unique, if you’re pushing it) trees and plants can be generated from simple (and in this case appropriately named) seed values.