From a clip I can no longer find on YouTube (I think it was there), presumably based on an Earthworm Jim cartoon. Here is Peter Puppy transforming into his monster version.
Traced and colored in Paint Tool SAI.
These are traced, both using Paint Tool SAI (trial version). The one on the left was using the pencil at 5px with size pressure on. The one on the right was on a “linework” layer using the Curve tool at 1px thickness (so no pressure). I prefer the left, but it helped to bring them side-by-side to see the difference. They’d probably a little more alike with an increased thickness for the one on the right using the Curve tool, but I think I’ll stick with using pencil with pressure on nonetheless.