The New Yorker wrote an extensive biographical piece on Guillermo del Toro that includes some interviews with the filmmaker where he discusses his upcoming projects At the Mountains of Madness and Frankenstein.
The article details the monster design for Frankenstein:
"In accordance with Mary Shelley's description, the head appeared to have been stolen from a cadaver: there was exposed sinew around the jaw, and the cheekbones looked ready to poke through the scrim of flesh. Most appallingly, the Creature lacked a nose; a single bridge bone protruded over an oval breathing hole. [The artist] had been etching deep furrows into the Creature's forehead, and shaved bits of clay were scattered on his desk, like clippings on a barbershop floor."
The Shoggoths had a racecar sheen. "They are pristine," he said. "They are functional. They are not asymmetric. Symmetry is efficiency. And these guys need to be efficient." He wasn't sure yet if the Shoggoth palette should be "pearlescent" or "circulatory"-reds and blues. Since the Shoggoths could mutate into anything, there was no fixed silhouette, but many would feature a "protoplasmic bowl," an abdomen-like area from which new forms could sprout. One maquette was a disorienting twist on classic Lovecraftian form. It looked like a giant octopus head with tentacles jutting from the top and the bottom-a fearful symmetry. "That's my belly in the middle," Guillermo del Toro joked.
In another maquette, the Shoggoth had sprouted two heads, each extending from brontosaurus-like necks. Their skulls could be smashed together to destroy victims. "The idea is to create craniums that function as jaws," he said. The Shoggoths would often create ghastly parodies of human forms; as they pursued the humans, they would imitate them, imperfectly.