In 1996, FASA ran into a licensing problem that has since become famous. When the game was first created in 1984, eight of the core mecha were based on designs from such anime as Super Dimension Fortress Macross and Fang of the Sun Dougram (yes, the Marauder does resemble a Zentradi Officer's Battlepod). The Macross designs were legitimately purchased and were being used legally. However, other rights to the actual shows and that IP in the United States had been transferred to Harmony Gold, a Los Angeles-based real estate and TV firm, to be used to create the first third of the TV series Robotech. The situation seemed to be one of "live and let live" until the mid '90s, when FASA contracted with Tyco Toys for a line of action figures, and also began to produce a cartoon series. Around the same time, Harmony Gold sublicensed its share of Robotech to Playmates Toys as a tie-in to the Exo Squad line. Playmates also created a toy for the line that bore a stunning similarity to a MadCat 'Mech, whereupon FASA naturally sued, the Mad Cat being something like the Beholder of BattleTech - an emblematic part of the IP that the FASA crew used heavily in promotion and intended to defend vigorously.
Harmony Gold promptly countersued, citing the use of Robotech mecha in their game, and suspecting that FASA was going to use the designs in their upcoming cartoon and toyline. The actual outcome of the case has been sealed, so no one who was not actually involved in the case really knows who won and who lost. However, FASA was sufficiently shaken by the experience to unilaterally stop using not only the Super Dimension Fortress Macross mechsnote but the Dougramnote and Crusher Joe note derived mechs as well, and any other design not created by FASA themselves. This ended in the odd situation of having to ban the designs created for FASA by Studio Nue (who had created Super Dimension Fortress Macross) so that BattleTech could be exported to Japan while not stepping on any toes. These 'Mech designs became known as the Unseen. The stats were (and still are) valid and legal for game purposes, but the ban did not allow any artwork or new miniatures to be made. This, of course, was especially devastating because the list of 'Mechs was more or less a who's-who of the most popular 'Mechs in the entire game - the Marauder, Warhammer, Archer and Battlemaster in particular were some of the most beloved 'Mechs of all time among the fans, and the Warhammer and BattleMaster even had something of a special place in BattleTech fiction: a tricked-out Warhammer being Natasha Kerensky's personal ride, Hanse Davion and the Red Corsair piloting BattleMasters.
In 2010-2011, Catalyst Game Labs tested the waters, believing that they could use the designs again, but backed off on the Super Dimension Fortress Macross 'Mechs after the legal team at Topps advised them against it. The other 'Mechs for a brief time were allowed a return to artwork, but after concerns about making miniatures gave Topps jitters, Catalyst was advised to not use that art either. There was also an abortive attempt at redesigning the 'Mechs in 2003 (in Technical Readout: Project Phoenix), but the redesigns were so badly received that later works ignored them.
We were at this post-1996 status quo until late July, 2015, when Catalyst finally decided to cut the Gordian knot altogether and redesign the Unseen so that they strongly resemble the originals but are legally distinct enough to be able to use without risking Harmony Gold suing them into oblivion. Much happy-dancing and reforming of the post-Unseen Broken Base was to be had in fandom circles. It also resulted in one of the very few out-and-out retcons actually acknowledged as such by the developers, as these new designs now completely replace the unusable designs in-universe.
However distinct the designs were, Harmony Gold did attempt to sue over them. In March 2017, Harmony Gold filed a complaint against Catalyst, as well as Piranha Games and Harebrained Schemes (makers of current BattleTech computer games, using original artwork adapted from the tabletop game) for infringement. Catalyst failed to defend itself at all, causing a judge to rule in Harmony Gold's favor by default, a decision that was later voided by mutual agreement of all parties at the time of settlement. The complaint against Harebrained Schemes was the most nonsensical, as the filed complaint against HBS claimed their designs too closely resembled Macross robots, despite being plainly based on other sources entirely (cited examples include the Atlas, based on an original BattleTech design, and the Shadow Hawk, inspired by a completely different anime franchise, neither of which even remotely resembled the alleged infringed designs). A jury trial was set for September 2018, amid speculation that Harmony Gold had finally met its legal match in PGI, who introduced evidence into open court from a 2017 arbitration agreement between Harmony Gold and Tatsunoko that stated that Harmony Gold did not have the copyright to the Macross designs, followed by a move by PGI to dismiss with prejudice on those grounds. Legal back-and-forth ensued, including the presiding judge threatening to impose severe legal sanctions on Harmony Gold's lawyers for insufficient attempts to rebut PGI's claims. On April 9th, 2018, Harmony Gold's case against Harebrained Schemes was officially dismissed with prejudice. On June 21, 2018, it was announced that Harmony Gold, in an agreement with PGI, had officially filed to dismiss with prejudice the cases against the remaining defendents, PGI and IMR, meaning that the lawsuit is officially ended, with Harebrained Schemes, PGI and Catalyst presumably free to use the new artwork as they see fit.note
The MechWarrior Online and BattleTech video games have since inserted partially redesigned versions of 'Unseen' designs in their 2018/2019 expansion packs, including the Phoenix Hawk, Rifleman, Archer, Warhammer, and Marauder, five of the most popular and distinctive designs affected by the legal troubles. This seems to suggest that Harebrained Schemes, PGI, and Catalyst are no longer concerned by Harmony Gold's interference in the matter, to the point that Harebrained Schemes openly and proudly announced that all BattleTech players would receive the Warhammer and Marauder update in the game for free to celebrate the release of their last major DLC package. Concurrently, by the spring of 2020, nearly all of the original 11 Unseen (except for the Scorpion and Goliath note ) had had their redesigns revealed as part of the Clan Invasion Kickstarter campaign by Catalyst.
Also, in an unrelated bit, when the game was first released in 1984 it was known as "BattleDroids" but since Lucas owns the word Droid it got into legal trouble over that name so in its next edition it was known as BattleTech and has been that ever since. Depending on who you talk to, this was done either because Lucasfilm sent a cease and desist letter over the use of the word "droid", or FASA decided to change the name on their own initiative in the hopes of getting on Lucas' good side while trying (and ultimately failing) to get a license to do a Star Wars roleplaying game.