The Big Bad is a term originally used in the Buffy the Vampire Slayer series created by Joss Whedon, and applied mostly by fans but also by the show's creators, to describe major villains in a particular season. Since then, it has been used in popular culture to describe annual villains in a television series, and has even been used in-universe by Jessica Jones to describe Kilgrave, the actual "Big Bad" of her TV series.
The Dragon is less common as a term, though it is almost as usual as a Big Bad in popular fiction. It is used to describe the Big Bad's main enforcer or henchmen, usually posing a much more physical threat to the protagonist than the actual Big Bad.
The television series set in the Marvel Cinematic Universe, Agents of S.H.I.E.L.D. in particular, seem to follow this particular tropes, and those are the big bads and their dragons in the different seasons/storylines.
- Big Bad: Kilgrave
- Big Bad: Iron Monger
- Big Bad: Abomination
- Big Bad: Whiplash
- Big Bad: Red Skull
- Big Bad: Yellowjacket