Unseen Prototype

Whenever someone decides to make an invention, they usually don't get it right the first time. Sometimes, it may result in Super Prototypes that have to be scaled down and depowered for easy mass-production capacity. Other times, it may go wrong. Horribly, horribly wrong, resulting in a string of machines ending in Mk. VIII, Mk IX, until it's finally (relatively) safe. Then, when someone shows you the Mk. X, you ask what happened with 1-9?

And there you have the Unseen Prototype(s). They are rarely mentioned in story, but come to mind once you realize that to get this far in an invention, there had to have been a lot of failures.

Examples of Unseen Prototype include:

Anime and Manga

The first time one of Dr. Gero's creations appears in Dragon Ball, it's Number 8. The next time one shows up, he's gone all the way up to 17. (An Omake illustration shows the fate of the numbers in between—most of them were failures or considered too uncontrollable.)
The title character of Yuria 100 Shiki is her inventor's 100th attempt at creating a Sex Bot. A few early models appear, including a crude inflatable doll.

Literature

Perdido Street Station: In his research on flight, Isaac comes across mention of Calligine, a Remaker who managed to graft functional bird wings onto his back. He notes that there had to be a lot of tests on criminals to get all the nerve endings and such right.

Video Games

The previous tests of the teleporter in Half Life 2. "What cat?!" Though, things don't go quite as planned with it even when it is seen.
Subject Delta of BioShock (series) already qualifies as a Super Prototype, but the name "Delta" implies that there were three before him. Knowing what the people of Rapture were screwing around with, it couldn't have been pretty.
There have been at least 13 Murakumo units produced in BlazBlue. Only the 13th, 11th, and 12th ever appear on-screen.

Film

Plan 9 from Outer Space. What were the first eight plans?
In Lilo and Stitch, Stitch is Experiment 626. Most of the previous 625 experiments are never seen, although some of them do make an appearance in The Series.
The Time Machine in Meet the Robinsons, which has had several failed prototypes, one of which looks like a Dali painting, and the Mark I which has only a single screw left.
Doctor Evil calls his Tractor Beam "Preparation H", as preparations A-G were complete failures. Of course, this is all to set up yet another joke for Scott to laugh at.

Live Action TV

Star Trek: The Original Series episode "The Ultimate Computer". Doctor Daystrom comes aboard to oversee tests of his M5 computer.

Kirk: I'm curious, Doctor. Why is it called M-5 and not M-1?
Daystrom: Well, you see, the multitronic units one through four were not entirely successful. This one is.
And later on, after it starts malfunctioning...
McCoy: The M-1 through M-4, remember? Not entirely successful. That's the way Daystrom put it.

In The Dr. Steel Show Episode 2, Doctor Steel calls his hamster, "Hamster 65". In Episode 1, he had a previous hamster "experiment" (referred to elsewhere as Hamster 42), which had died. One wonders what happened to all the other ones?
Babylon 5. We do eventually find out what happened to Babylons One through Four.
The Benny Hill Show. Benny (as Fred Scuttle) is displaying his homemade, backyard rocket in which he plans to fly to the moon.

Scuttle: May I present...Indestructable the Second!
Interviewer: What happend to Indestructable the First?
Scuttle: It blew up.

Whisper 115 of Tower Prep seems to have a lot of these-such as the psychotic Whisper 023. What happened between 23 and 115? What happened before 23?!

Comics

Machine-Man in Marvel Comics, also known as "X-51" or Aaron Stack (made famous by Nextwave). We never really find out about the previous 50, though Nextwave shows a joking page about one of his predecessors who is a partly-human looking killbot who happens to be employed as a priest. Somehow.
- The original prototypes all suffered mental breakdowns. The reason? They had human-level potential but were treated as mere machines. Only their creator realized this so he took one home and treated it as a person. This is the one who became a superhero. (This is from the original story, not Nextwave.)
Marvel's Canadian government Super Team Alpha Flight has three training levels, the other two named appropriately Beta and Gamma. Gamma trainees graduate to Beta and then to Alpha. Though Beta eventually went rogue.

Webcomics

In Sluggy Freelance, this is a frequent joke about Riff's robots. First there was Mark I (which he built to do his laundry but was nevertheless capable of taking on an army), then Mark II (which fought Oasis). Then Mark V (which fought a possessed Gwynn and Hereti-Corp); Riff doesn't want to talk about Mark III and IV. After the robot's long absence from the comic, Riff revealed Mark XVII. We don't know much about the intervening models, save that one of them was made of cheese.

Western Animation

On My Life as a Teenage Robot, we actually do find out what happened with XJs 1-8.

Real Life

Real Life: WD-40, which stands for "Water Displacement - 40th Attempt."
Formula 409. So named because it took the inventors 409 tries to get the formula right.
- One of their TV ads makes fun of this by showing a "Formula 410" prototype: it melts right through walls.