Demogorgon is based on the invented, supposedly pagan god or demon Demogorgon, which was first spoken of by Christian scholars as a being whose very name is taboo. A creature named Demogorgon is featured in John Milton's Paradise Lost, Lodovico Ariosto's Orlando Furioso, Spenser's The Faerie Queene and Percy Bysshe Shelley's Prometheus Unbound, although the D&D Demogorgon's appearance and history may not be based on any of these sources.
This is a personal bias, but whenever I see imagery such as:
Lovecraftian influence leaps to the forefront; tentacles, immense size, terrifying presence, bestial integrated - yet alienist... tagged to represent that bias.
Are there sourced statements about what inspired the D&D Demogorgon visuals?