Can't help with the cartoon character, but Lyne has a free Great Dane morph for the Poser dog at PlanIT 3D if you want to try a more realistic look.

artistheat made a Scooby character for a few renders; I'm not sure which figure was mutated, however.

Teyon produced a great Scooby model in Rhino 3D; perhaps he can be coerced to post it in Free Stuff in .obj format so that others can rig it for posing.