Monster Delay

When the Big Bad is a monster -- especially a large monster -- it is imperative to avoid showing us the monster for as long as possible.

It is OK to show small portions of the monster (tails, claws, etc.) earlier; the filmmaker should be toward revealing such a monster as an exotic dancer is toward removing her clothing. But the full reveal of the monster should take a long time—at least several episodes on television; at least twenty or thirty minutes in film.

The larger and badder the monster, the longer it will take for them to become visible.

This law emerges from the cost of special effects and the desire to keep the audience in suspense until the "good stuff" appears. It has become a standard feature of monster movies.

Conforming to this law often involves extensive use of reaction shots, shadow shots, or shots of the monster that are obscured by smoke, waves, darkness, blood, etc.

Of course, actually showing the monster usually heralds the decline of its earlier invincibility.

Examples of Monster Delay include:

Anime and Manga

Pluto The title character is only glimpsed at until his appearance 7/8ths of the way through.

Comic Books

Galactus. In Fantastic Four #48, Galactus, who is probably the largest and most omnipotent bad guy in the Marvel pantheon (at that time), does not appear until the very last panel.

Fan Works

Daemon in the Fan fic Tamers Forever, lampshaded by the author.

Episode five and Daemon hasn't arrived to Shinjuku… *sigh* Oh well, I just hope I actually get him in there before chapter eight, and I expect to finish this story in twenty chapters in the worst case scenario.

Film

Godzilla. The quintessential example. A monster the size of a skyscraper manages to travel halfway around the world while being stalked by the US military, attacks Manhattan, and yet still does not fully appear on screen for 45 minutes.
King Kong. Justified in that the main characters travel for a long time in order to see the monster.
Jaws. Further justified in that Spielberg didn't like the model of the shark used in the film, so he ensured as little of it was shown on camera as possible.
Alien, with the added bonus of including only 3 jump scares in the entire film...and only one of them has anything to do with the eponymous xenomorph.
Cloverfield. Very closely related to the godzilla example, we don't get to see the entire thing until at LEAST half the movie has gone by. Not only is it's appearance rather hard to explain without seeing the movie, it's quite ugly too.
Predator: Only the view of the soldiers from the predator's eye-view, then a view of the cloaked predator, then close-ups of him patching up his wound, a full-body view, and finally the unmasking.
In Terminator 2: Judgement Day, the monster is visible (as Robert Patrick in police blues) from the beginning, but its superpowers (and stunning visual effects) are trickled in as per Monster Delay. The powers are hinted at with first encounter, their lethality is realized with the death of the foster parents, and they're fully exploited to the limits in the double-finale.
Averted by Rawhead Rex; it was Clive Barker's intent to make the monster as visible as possible early in the film.

Live-Action TV

Buffy the Vampire Slayer. The Mayor's ascension takes an entire season arc.
Firefly. The reavers are constantly referred to, but hardly ever appear on screen.
The smoke monster on Lost was not seen in full until the middle of season 2, and his nature wasn't fully understood until season 6.

Video Games

In Final Fantasy X, we see bits and pieces of Sin, but not the entire thing until its third appearance. Interestingly, its earlier glimpses make it look much more like an Eldritch Abomination than its full body, which is something like a blind whale with extra designs on it.

Web Comics

Order of the Stick has the literal, and so named, Monster in the Darkness. Even the monster himself does not know what he is.

Western Animation

Parodied in an episode of Doug. Doug watches a horror movie called The Abnormal, about an evil alien shape-shifter whose true form is always just off-screen. Doug can't bring himself to watch The Reveal near the end of the movie, and ends up having reoccurring nightmares about it. He finally works up the nerve to watch the film one last time... and discovers the monster is just a guy in a cheap suit with an obvious zipper on the back.
In the Ed, Edd 'n' Eddy episode "The Day the Ed Stood Still", Ed's friends dress him up like a monster, and Ed proceeds to get a little too into character and go on a rampage. We don't see Ed's monster costume in full until about two-thirds of the way into the cartoon.