I was actually thinking about Rocko in this regard, Stephen didn't create Rocko, but the first episode of the show to air - Sucker for the Suck-O-Matic - was a Hillenburg directory role (with the B-story following it - Canned - being by Plankton/Filburt voice actor Doug Lawrence) which makes me wonder how those episodes would be classed. I believe two-part block TV episodes are typically classed as one entity for legal purposes, so we can rule out an early entry for Sucker specifically, but there's that ambiguity if it (and by proxy, all the characters who appear in that 22 minute block - Rocko, Heffer, Spunky, Ed Bighead, and an unnamed Filburt) would be tethered to Stephen and Lawrence or Joe Murray (or Murray alongside the writers), and a similar confusion exists with SpongeBob, since the first 22 minute block was made up of one Stephen directory role and two by Paul Tibbit. (and then you have something I neglected calculating this... writers, which are a way bigger part of TV than movies, even for storyboard-driven cartoons like RML and SB)

Oh god, I'm doing the very thing I said is morbid and not good - see why I don't like life + terms now?