7 Doom characters were built as sculptures during Doom & Doom II development. The first models - Doomguy, Baron of Hell & Cyberdemon - were sculpted by Adrian Carmack. Working on them proved to be a lot more work then he expected, which is why id eventually hired Gregor Punchatz to build the rest of the models (Arch-Vile, Mancubus, Revenant & Spider Mastermind).
Getting the modeled characters into the game was a relatively straightforward task: The sculpts were first captured from 8 different angels using a video camera. The scans were then finished by rescaling, recolouring and animating them in the Doom engine's 256 colour graphics format. All in all this procedure saved Adrian and Kevin a lot of trouble, as they didn't have to draw but a few character sprite sets from scratch.