All of the used and unused Type combinations as of Gen 7!
(the faded ones are combinations that have only been represented by Legendaries, Forms or Megas)
Fire/Grass has to be a burning bush. Ghost/Rock should be one of those bug-eyed statues.
Fire/Ice should be a flaming ice cube.
Fire/Fairy needs to be a regional variant of Rapidash that is an actual magical unicorn.