Y'know, I'd have to say that any of the bosses from Dynamite Headdy were qualifiably weird.
You've got slightly strange bosses, like the "Air Battleship", which is just a normal battleship being hoisted into the air by a crane, or Tarot, a crazy-faced sun (with a broken facade) who can only be hurt by the little planetoids orbiting him, and then you've got the incidentally-random, like fighting Trouble Bruin as a group of teamsters pulls a Japanese folk art painting onto the background (as the old backround toppled over), or fighting a giant mechanical dog on wheels as a visible orchestra plays a song from the Nutrcracker in the background.
And then there's the outright-bizarre, like a wooden mannequin that attacks by wearing a veriety of costumes, or a giant mechanical-ish head that transforms through the stages of its life (from baby to boy to man to old man, each encased within the last), or the dual-faced "twin freaks", whose mood and actions depend on which way the level is current flipped.
Throw in Trouble Bruin's contraptions that roll, climb, grapple, or bisect towers, and you've got a whole mess or bosses that you can't explain without sounding a little crazy.
Honestly, I think the last boss of that game is one of the most mundane. Except for the fact that he wore a little smiley face on a chain around his neck.
