Toying with a very different layout for the Unseen article... but I'm starting with a copy of the latest version that I edited.
Unseen refers to certain imagery, namely the visual appearance of a number of iconic BattleMechs, that has not been depicted in official publications since 1994 for legal reasons.
To wit, only their visual appearance may not be officially used anymore; the units as such (particularly their names and game stats) remain part of the BattleTech canon and are even frequently mentioned in novels and fiction. Neither the images nor the units they represent were retconned out.
The term is alternatively used to summarily refer to the real-world imagery as such, to the particular in-universe designs (and subvariants) in question, or as an adjective.
The list of "Unseen" has repeatedly changed over time, with some individual designs put on and removed from the list several times.
When FASA created the BattleTech boardgame in 1984, the designers licensed the rights to use pre-existing Mecha from other IPs, namely the Japanese anime series' The Super Dimension Fortress Macross, Fang of the Sun Dougram and Crusher Joe as visual representation of several 'Mechs and other units within the BattleTech universe (under new names and with BattleTech game stats devised entirely by FASA). However, during a series of lawsuits in 1994 FASA found out that Twentieth Century Imports (TCI), from whom FASA's rights to the images derived, might not have been in a legal position to provide these rights to FASA in the first place. The lawsuits were settled out of court.
As part of the settlement agreement, FASA forfeited the right to use the images henceforth. As this affected many core designs which have been described as the bedrock of the BattleTech universe at the time, this was a serious blow to FASA. All existing products featuring pictures of the "Unseen", be it on the cover or in the rulebooks—effectively most if not all BattleTech publications—had to be discontinued, and while numerous other designs remained that could still be depicted, this massively affected the BattleTech line.
Beyond the specific artwork covered in the "Unseen" lawsuits, FASA decided to treat all designs as "Unseen" that had been developed out-of-house, such as the designs made for FASA by Victor Musical Industries, Inc. As a result, there are two unofficial categories of "Unseen" images:
- Images to which FASA found they might never have legally acquired the rights to, and which they are not allowed to use or show;
- Images which were never contested and to which FASA did obtain the rights, but which, being out-of-house artwork, are not used on general principle.
In 2003, Technical Readout: Project Phoenix was published that postulated upgrades and newer versions for most of the "Unseen" 'Mechs from the in-game year 3067 forthwith. Dubbed "Reseen," these evolutions were sufficiently visually different from their parent designs that they could be depicted. Technical Readout: 3055 Upgrade continued with "Phoenix" upgrades for the remaining "Unseen" 'Mechs, and the "Reseen" Samurai first appeared in AeroTech 2 Revised Edition and then in Technical Readout: 3039.
To wit, the "Reseen" did not replace the "Unseen" or retcon them out. They are new, additional designs with new art, meant to gradually replace the "Unseen" as the timeline progresses.
The term "Reseen" is also applied to designs that were "Unseen" at one point, but were taken off the list again. For the purposes of this article, these designs will be referred to as "returned."
25 Years of Art and Fiction
On 24 June, 2009, it was announced that Catalyst Game Labs would resume use of the original "Unseen" artwork for these designs. The announcement did not mention whether or not Catalyst Game Labs had secured the permissions needed. 
Shortly after the return of the "Unseen" was announced, the BattleTech: 25 Years of Art & Fiction PDF was pulled from circulation. Managing editor Randall Bills made a statement on Catalyst Game Labs' website on August 10th, 2009 explaining the move. Twelve designs, originating from Macross, were again pulled from circulation and will remain "Unseen", due to their exclusive distribution rights (outside of Japan) belonging to another company (Harmony Gold).
Since the article was written, further clarification was given that most of the images previously declared "Unseen" would remain "Unseen" in order to avoid the possibility of future legal issues.
During this period of time, "Unseen" images were added to several other products in the production queue and were not removed prior to initial distribution. This was not a declaration that the images would be used again and they will be removed when the product is updated. These products include Record Sheets: Operation Klondike, Historical Turning Points: Galtor, Historical: Reunification War, and Record Sheets: 3055 Upgrade.
(Table used for organization. The intention is to show the basics of which images were removed and revised, not to get into the specifics on variants. Decreasing thumbnail size will allow both revised Longbow images to be displayed.)
|Name||Removed Image||Revised Image||Name||Removed Image||Revised Image||Name||Removed Image||Revised Image(s)|
|Archer||Battlemaster||Black Python (Viper)|
|Conjurer (Hellhound)||Crusader||Galleon Light Tank|
|Glass Spider (Galahad)||Goliath||Grand Crusader|
|Griffin||Griffin IIC||Horned Owl (Peregrine)|
|Howler (Baboon)||Incubus (Vixen)||Jenner IIC|
|Locust||Locust IIC||Longbow ("removed" image is placeholder)|
|Marauder||Marauder II||Marauder IIC|
These images, from the 3025 through 3058 eras, are no longer depicted in official publications:
Origins of the "Unseen"
Below, the "Unseen" are divided into categories based on their original series or creator. Each unit is listed by its BattleTech name first, followed by the model and/or name used in its original series (where applicable).
Super Dimension Fortress Macross
- VF-1 Valkyrie
This Mecha from the Super Dimension Fortress Macross series normally transformed into the three combat modes recognized in BattleTech by Land Air 'Mechs. Within the BattleTech game this one vehicle was split into distinct non-transformable 'Mechs models and LAM models. The LAMs, true to their artistic origin, could transform into aerospace fighter, hybrid, and 'Mech mode.
- Wasp - VF-1S
- Wasp LAM - VF-1S
- Stinger - VF-1A
- Stinger LAM - VF-1A
- Valkyrie - VF-1S
- Phoenix Hawk - VF-1S Super Valkyrie
- Phoenix Hawk LAM - VF-1S Super Valkyrie
- Crusader - VF-1A Armored Valkyrie
- Rifleman - ADR-04-Mk.X Destroid Defender
- Archer - MBR-07-Mk.II Destroid Spartan
- Warhammer - MBR-04-Mk.VI Destroid Tomahawk
- Longbow - SDR-04-Mk.XII Destroid Phalanx
- Zentradi Tactical Pods
- Marauder - Glaug Commander Type
- Marauder II - Glaug Commander Type
- In December of 2011, BattleTech's line developer ruled that the Ostscout, Ostsol, and Ostroc were sufficiently different from their inspiration (Regult Scout Pod, Regult Battle Pod, and Regult Heavy Missile Pod, respectively) to be removed from the "Unseen" list.
- Locust - Ostall
- Samurai AeroSpace Fighter - TR-5 Harpy
- Galleon Light Tank (3025) - Galleon
- The Technical Readout: 3058 version of the Galleon was ruled, in December 2011, to be similar enough to the original that it should be removed, however, that decision was reversed in November 2014.
- Other units of note
- While inspired by Crusher Joe images, the following have never been "Unseen":
- Sparrowhawk AeroSpace Fighter - TR-5 Harpy and SR-5 Siren
- Corsair AeroSpace Fighter - SR-5 Siren
- Leopard Dropship - Minerva
Fang of the Sun: Dougram
- Shadow Hawk - Dougram
- Griffin - Soltic H8 Roundfacer
- Scorpion - F35C Blizzard Gunner
- Wolverine - Abitate T-10B/T-10C Blockhead
- Thunderbolt - Hasty F4X Ironfoot
- Goliath - Abitate F44A Crab Gunner
- Battlemaster - Soltic HT-128 Bigfoot
Victor Musical Industries, Inc.
These images by Victor Musical Industries, Inc. were created for BattleTech but were removed to prevent the possibility of future legal issues. Note that while the IIC images (with the exception of the Jenner IIC) are obviously inspired by images taken from SDF Macross, they were removed only because they were created by an "out-of-house" source.
- Grand Crusader
- Howler (Baboon)
- Locust IIC
- Incubus (Vixen)
- Jenner IIC
- Horned Owl (Peregrine)
- Griffin IIC
- Shadow Hawk IIC
- Conjurer (Hellhound)
- Vapor Eagle (Goshawk)
- Glass Spider (Galahad)
- Rifleman IIC
- Black Python (Viper)
- Warhammer IIC
- Phoenix Hawk IIC
- Marauder IIC
- Stone Rhino (Behemoth)
- Bane (Kraken)
- Catalyst Game Labs website, "Catalyst Game Labs Brings Back “Unseen”"
- Catalyst Game Labs website, "Sometimes Things Just Don’t Go As We Want Them To"