I read the article about Buck Nix/Old Doc Yak, and I thought you might like to know that he has indeed been seen later than the 1930s; in 1998, to be specific, in a one-shot comic book from Shanda Fantasy Arts entitled Valiant Varmints.

SPOILER ALERT

In this book, a mysterious figure named Bullethead is going after a mysterious item, and he's arranged to keep the Valiant Varmints -- a team of anthropomorphic heroes including Bullet Crow, Phantom Bunny, Fission Chicken, Beatrix Farmer, and Superswine, among others -- occupied while he goes after it. He turns out to be Doc Yak, who just wants his old license plate back, after having lost it decades ago to The Gumps. The story ends happily for everyone, including Yak and Yutch.