A series of very graphic adult Gravitation doujinshi by the original mangaka.

Megamix

Capybara カピバラ (Eiri x Shuichi / Hiro x Suguru ) (2007)

Panda パンダ (Yuki uke, Seguchi x Yuki x Tatsuha) (2006)

Kumagorou くまごろう (Shuichi x Yuki) (2006)

Zebra ?