18+ Akaaka to Shita Chi no Monogatari

Itsuki and Hayato, two brothers who grew up in a village nestled deep in the mountains, set out to hunt down a wolf that has been attacking the livestock of the village. Itsuki is about to go in for the kill when a gigantic beast appears out of nowhere, snatching up the wolf in its jaws and sucking the life out of it. The beast that stared Itsuki down was a gargantuan, blood-sucking vampire deer... Will the two brothers be able to return to the village alive?!

(Source: Manga Box)