Villains who are referred to in the very titles of the works in which they appear, whether they are Protagonists or simply titular antagonists (i.e. Macbeth, Jaws, the Animator, Diablo, Siren Head, The Fallen, Freddy Fazbear, Carmen Sandiego, etc.). While this does not always happen, sometimes a protagonist will be mentioned in the same title (especially a main protagonist or the one they are fighting against), or even other villains.

This can also apply to villains who come from TV shows or other series, and are the titular character for any episodes or chapters (i.e. Cassandra, Katz, Horde Prime, Eric Cartman, Janja, The Lich, Gunmar, etc.). 

Titular villains tend to be the main antagonist of the work (or just episode/chapter) they are from. If they are not the main antagonist, they still tend to be very significant anyway.

Titular and Eponymous are closely related and similar but not always interchangeable:

• Eponymous means the title contains part or all of a character's name, making them eponymous. Ex. Fantastic Beasts: The Crimes of Grindelwald; Gellert Grindelwald is eponymous, as part of his name is in the title.
• Titular means a character is referred to or referenced by the title without using the character's name. Ex. The Man with the Golden Gun; while his name is not mentioned, Francisco Scaramanga is the character the title refers to.