Watcher, at least for its appearance in Shin Megami Tensei, seems to be inspired by a character known as the Eye from Kaoru Kurimoto's Makai Suikoden novel series, along with a sketch by Odilon Redon (presumably "The Eye, Like a Strange Balloon, Mounts Toward Infinity").[1]
A monster known as "one who watches." It takes the form of a giant eyeball that floats in the air. It is elusive and spies on people for the angels as one of their informants.