Demogorgon - Prince of Demons
I drew this image to supplement an AD&D adventure that I was creating. The being is from Monster Manual I, Gary Gygax: "Demogorgon appears as an 18-foot tall reptilian-humanoid. He has two heads that bear the visages of baboons. His blue-green skin is plated with snake-like scales, his body and legs are those of a giant lizard, his twin necks resemble snakes, and his thick tail is forked. In place of arms, he has two huge tentacles."
While MMI indicates that this demon has baboon heads, I decided to model them more after baboon skulls to enhance the general evil look of the thing. Keep in mind that a human to scale to this drawing would only come roughly to the middle of its thighs.