Is that last page part of a manga?

No

Yes it is, it’s called the beast and the monster. I don’t think you can find it on mangago tho, here’s the link to myreadingmanga: https://myreadingmanga.info/renji-range-beast-monster-eng/
Only two chapters are translated

Never mind, found it on mangago: http://www.mangago.zone/read-manga/the_monster_and_the_beast/

Thanks

Oh i thought you were asking if this manga only had one chapter lol sorry the person above is right