The main difference is the inclusion of bestiality content. Regular hentai comics typically focus on human-human relationships or fictional creatures within certain boundaries. Beastiality hentai comics involve inappropriate and unethical themes involving animals, which is illegal and unacceptable in many places.
Doujin hentai comics often have a more niche or specific focus and are usually created by independent artists or groups. Regular hentai comics might follow more mainstream themes and be produced by established publishers.
The main difference is that doujin hentai comics are often created by amateurs or fans, while regular hentai comics are usually produced by professional companies or artists.