This is JC The Slug (my avatar) on one of those Cel Sheets that was commonly used for old-school hand-drawn animations.
I drew the Slug with Grease Pencil and applied some colour and ink filters using the HSV Modifiers, I even added a shadow to give off the effect that it was drawn on a cel and some colors bleed out to replicate the effect of paint added on the reverse side of the sheet.
The Cel Sheet effect was done with Inkscape.
I also made this quick test animation of the Slug.