http://www.mangago.me/read-manga/zion_no_koeda/an/zion-no-koeda-chapter-extra%203.html/1/ ...

I saw another drawing with a similar style and it was just an illustration - this may well be the same - but I can't say for sure.

Most likely just an illustration by Ayumi Kasai

Ayumi Kasai's illustrations
http://www.mangago.me/home/photo/10816/

Thank you very much. Now I'm sad :(