Enter Eponymous

It's fairly common in TV series to title an episode that introduces a new recurring or main character with Enter (character's name) or Enter the (attribute/title/race of the new character). This has its roots as a Stage Direction, so it is on the minds of writers more than viewers.

In anime, this can instead be "sono na wa", "His Name Is". (No, not that His Name Is--.) Related to I, Noun.

Examples of Enter Eponymous include:

Anime and Manga

Ranma ½ had a few of those: "Enter Mousse! The Fist of the White Swan", "Enter Ryoga! The Eternal 'Lost Boy'", "Enter Kuno, the Night-Prowling Knight" and "Enter Gosunkugi, the New Rival!?", as well as "Enter Shampoo, the Gung-Ho Girl!"
Inuyasha: "Enter Shippo, Plus the Amazing Thunder Brothers".
The first disc of the Naruto dub-only DVD series is called "Enter: Naruto Uzumaki".
Digimon dub examples: episodes 1.40, "Enter the Dark Masters" and 2.1, "Enter Flamedramon."
Pokémon: Japan has used "Enter Champion Shirona" (Cynthia) and "Enter Contest Master Mikuri" (Wallace). In between those episodes, the episode for Ash's first confrontation with the region's villainous team was called "Their Name is Team Galaxy" in Japan while the dub went with "Enter Galactic".
- Suffice to say, the Japanese titles do this a lot. During the show's long history, this format has also been used for the first appearances of the Squirtle Squad, Bruno, Drake's Dragonite, Gligarman, Morty, Dr. Abby, The Phantom Masked Coordinator, Savannah, the other Drake, Fantina and Jun.

Examples of the "His Name Is" variant

GaoGaiGar did it repeatedly. First they had "His Name Is ChoRyuJin", then later "His name is GekiRyuJin".
Code Geass had "His Name Is Zero."
Fullmetal Alchemist: Scar's last episode. Its title is "His Name is Unknown".
Gundam Wing subverted this translation pattern: Sono na wa Epyon became the ominously prophetic-sounding And Its Name Is Epyon in the English dub.
Ginga Densetsu Weed had "His Name Is Weed" as its first episode.
Dragonball Z had "His Name is Cell" right after the standard "Who is this new threat" at the end of the previous episode.

Fan Works

Enter the Dragon, a Harry Potter Alternate Universe/Crossover fic by Doghead Thirteen deliberately copies the Trope Namer below -- but is far more literal in its application.

Film

Enter the Dragon - possibly the Trope Codifier. Many of the examples on this page are take-offs of this particular title.

Literature

A chapter of the English translation of The Master and Margarita is called "Enter the Hero," because it's nowhere near the first chapter of the book.
A Sherlock Holmes pastiche about his brother Mycroft is called Enter the Lion.
Fran Drescher's autobiography spoofs the trope. It's called Enter Whining.
A chapter of Moby Dick: "Enter Ahab; to him, Stubb".

Live-Action TV

The first episode of The Lone Ranger is titled "Enter the Lone Ranger", showing the age of this trope.
Dynasty had "Enter Alexis".

Professional Wrestling

ECW wrestler The Sandman always made his entrance to Metallica's "Enter Sandman".

Video Games

The first mission in Star Fox 64 is called "Enter Starfox" and the mission that introduces Star Wolf is called "Enter Star Wolf".
Disgaea's second chapter: "Enter Flonne"

Western Animation

The 1980s version of Teenage Mutant Ninja Turtles had three such episodes: "Enter the Rat King", "Enter The Shredder" and "Enter the Fly".
X-Men: The Animated Series had "Enter Magneto".
Spider-Man: The Animated Series from the 1990s had two: "Enter The Punisher" and "Enter The Green Goblin".
- The second PlayStation video game was subtitled "Enter Electro".
Gargoyles had "Enter Macbeth", which is appropriately Shakespearean.
Jackie Chan Adventures had "Enter the Viper" (introducing Viper), "Re-Enter the J-Team" (reintroducing said team), and "Re-Enter the Dragon" (a reference to the first example of this list.)
The Transformers Generation 1 episode "Enter the Nightbird", starring the eponymous one-shot antagonistic newborn female giant robot ninja.
The BOTS Master introduced Ninjzz in the second episode aptly titled "Enter the Ninjzz".
The Swat Kats episode "Enter the Madkat".