The Half Life 2 Headcrab (2016) was the main feature of my Dr. Issac Kleiner (and Heady Lamarr!) cosplay, and one that got me into making creatures.
At its base, it’s an old polystyrene bike helmet – which means it’s designed to fit heads well. I used a number of cans of expanding foam to create the shape of the body, and created the front and back legs, and the “mouth” seperately. A knife was used to carve them into shape and boy, does it produce a lot of scrap foam. All the limbs were glued on and then blended in with more expanding foam and carving.
Once everything was shaped correctly, the foam was covered in a few layers of caulking gap filler to fill in the porus surface. After a primer layer, a variety of (in hindsite, enamel) spraypaints were used to paint the surface.