This was probably the most difficult custom I've ever made, not to mention time consuming - 2 1/2 years from start to finish! I was originally going to use the Little Nicky bulldog that most people use for Lockjaw, with the intention of adding articulation. When I got it I found it was hollow, and thus useless - I would have to build him myself!
He's essentially a full sculpt over a fodder skeleton with resin cast feet. Most of the joints were transplanted from other figures, but the hips (rear shoulders? Whatever they're called on a dog) and ankles were scratchbuilt. All in all, he's got ball & socket ankles & torso, cut shoulders and a swivel/hinge neck - more than enough.
He's not perfect - I'd love to go back and make an improved version, if I had the time & money - but he's good enough for my display!