The Lab (Dr. Muto [2002])

[Will insert description here later; my apologies for not being able to finish this list in time without cutting corners a little]

(Length before loop: 3.5+ minutes)

I have no idea why every single mascot platformer wanted to have transformations, but they did. Dr. Muto plays that very straight. You're a mad scientist and morph into stuff that makes your fly, swim, crawl, punch. Doing that regularly would just be boring.

Fun fact: This was the last game from Ed Logg, creator of such classics as Asteroids, Gauntlet and Centipede. What a way to go out.