Mastiff

Mastiff was a type of cyborg deployed circa 2020. It was hulking and had its hands hidden in armor, with cannons on its wrists. Raiden, as a Maverick agent, managed to somehow find design specs relating to the Mastiff during or prior to the mission where he sustained severe injures. Maverick later retrieved his arm and located two blueprints of the Mastiff, one of which detailed its head, although they required passwords to get to the encrypted files, and the blueprints themselves ended up corrupted, due to the damage to Raiden's arm.

Behind the scenes
The Mastiff first appeared in the Metal Gear Rising: Revengeance E3 Trailer in 2010 (then known as Metal Gear Solid: Rising) as an enemy that Raiden ambushed and proceeded to cut up via Zandetsu, before absorbing its spinal cord energy and then crushing it (although it was not identified as such). It was based on designs made by Yoji Shinkawa that were originally intended for Metal Gear Solid 4: Guns of the Patriots, as a design for the Beauty and the Beast Unit.

The Mastiff was not given a name until after the "Make It Right: decryption" viral marketing site uploaded artwork of the Mastiff that required a command key code to access, with the in-universe explanation given on the Metal Gear Solid Facebook page being that Maverick located an encrypted file cluster that was password protected, and found clues to the password in the names "Anatolian", "Alpine", "Bully Kutta", "Cane Corso", "Bordeaux", "Pyrenean", and "Neapolitan", all of which are breeds of the mastiff dog breed.