The symbol (like ど) was proposed by Sakuragi Rian himself and made by Stylecase.

It is intended to be used to disambiguate doujin (どうじん) artworks from official ones.
It was proposed seemingly because of an incident where an actor of Kemono Friends' stage play mistakenly posted a picture of CarasOhmi's original great auk Friend on Twitter (correction tweet; screencap by Sakuragi Rian).