First rule of Rambo: replacing any story's protagonist with Rambo technically makes that a Rambo story.
Proof of concept: this summary of Rambo V, which, as ComingSoon has pointed out, is the exact same as the official summary for a James Byron Huggins' novel, Hunter, only with a find/replace done to replace the name Nathaniel Hunter with John Rambo:
John Rambo could track anyone - or anything - on earth. Now the military desperately needs him for a mission that his ultrasensitive instincts tell him he should refuse. A beast is loose somewhere north of the Artic Circle. It has already decimated a secret research facility and annihilated a squad of elite military guards. And the raging creature is headed south toward civilization, ready to wreak bloody devastation.
It's a job that Rambo and his 22-year-old hunting partner, Beau Brady, can't turn down, but they and a team of highly-skilled special forces kill team discover that the prey is a terror beyond their wildest imagination - a half-human abomination created by a renegade agency through a series of outlawed genetic experiments. It has man's cunning, a predator's savageness, and a prehistoric power that has transcended the ages. And even if Rambo and Beau survive its unrelenting hunger for human blood, they'll still have to contront the grim reality that it may have grown immortal.
Sly sent the above poster to AICN and, though it's an early design, thus far it appears the "half-human abomination" is being envisioned as a wall of hair with wolf eyes and a tooth-covered branch for a mouth. I suppose that's scary in an abstract way. Not so much an "Oh no, it's going to kill me!" scary. More of an "Oh, god, how does it eat without any obvious mouth orifice?" scary.