By Gad, I think Bup is right.
Picture of Seymour: See http://www.toymania.com/toyfair2000/living/krofft.shtml and scroll down about halfway to the eight action figures. Seymour is bottom row, second from the left.
The original Evil Grimace is Seymour minus two limbs, and with a color change.
So now we see the pattern. McDonald’s intended to borrow not merely the characters from Pufnstuf but the paradigm. Ronald and his comically inept assistants would fight a running battle against the forces of evil who try to steal their food. Hamburglar is cribbed from Witchiepoo and Evil Grimace from her henchman Seymour.
As the campaign unfolded, however, this scenario probably proved too dark for the very young target audience . . . so Grimace shed two more limbs and metastasized into a “fun-loving fellow” who enjoys, but does not steal, milkshakes. But he kept his name (sans “Evil”).
So what is the Grimace? He is:
(1) an anthropomorphic spider . . .
(2) changed in form just enough to stave off the inevitable lawsuit, and then . . .
(3) changed again to be less threatening and more fun-loving.