Calvin and Hobbes.
Under the Alter Ego section, there's a section "Animals" where it's mentioned that sometimes Calvin appears as various animals after being transmogrified. However, transmogrification is very different from the "Alter Ego" strips, in which the entire reality is changed: not only is Calvin Spaceman Spiff, but he's walking around on an alien planet; or not only is he a T-Rex, but he's in the Jurassic. Even when the world stays as Calvin's accepted reality (as when he plays Stupendous Man and burns up his school with a magnifying glass) there's still a difference in tone and perspective. Many times, even the drawing style is different. When Calvin transmogrifies into a tiger, however, the perspective of the strip doesn't change -- he's still Calvin, it's just that he's a tiger. He's even a cartoony, Hobbes-like tiger, not a realistically-drawn tiger (which would move it over into the alter-ego realm). I think that this is an important distinction because the "feel" of the alter-ego strips is always very different from the normal strips.