There are characters in the public domain that happen to wear diapers, like this Jerry-looking guy. His name is Marmaduke Mouse, and he’s from an obscure comic series from the 1940s. A bit more info here.
FWI, I first learned of Marmaduke Mouse from this fancomic by KelvinTheLion: Marmaduke X Superkatt
My sources:
Don Markstein’s Toonpedia
DC Database
Public Domain Super Heroes
Comic Books Plus (Hit Comics)
Comic Books Plus (Marmaduke Mouse)
A pair of Skeb commissions drawn by EndoudouhuRau for CyberDiaperMan.
EndoudouhuRau
Twitter Bluesky Misskey.io
Skeb Fantia pixivFANBOX
Pixiv
CyberDiaperMan
Bluesky Twitter Mastodon
Fur Affinity