It's the Great Pumpkin(head), Charlie Brown!
Half-way through this illustration it occurred to me that people may not recognize the monster in the background–and then, of course, not get the gag (such as it is) at all. Oh well, if so: sorry, folks! Anyway, that’s not the Great Pumpkin coming to greet Linus, but rather Pumpkinhead, the vengeful Stan Winston-designed monster from the 1988 horror movie Pumpkinhead.
This is from a project called Holloweirdos, which is posting daily Halloween-themed images throughout October. You can check out all of them so far here:http://wap.wideawakepress.com/