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A trademark is a name, phrase or symbol used by a company to distinguish their goods or services from those of other companies in a similar range, preventing "brand confusion". Trademarks are indicated by a ™ symbol, or in the case of registered trademarks, a ® symbol. Hasbro's ability to legally secure a trademark dictates what proper nouns they will use to describe a toy, gimmick, or line.
It is not the same thing as copyright.
Hasbro does not strictly need to claim trademarks for the character names applied to individual toys. Some toy companies do not claim them, though this is most often in the case of short-lived licensed lines based on movies or video games. But, Hasbro values the advantages of maintaining established characters (or character types) in the long term, and they wish to be able to hold exclusive claim to those names and use them as long as possible without fear of litigation from or loss to another company.
Trademark law is full of gray areas and complexity; otherwise lawyers couldn't charge exorbitant sums for their services in that field. The whims of the judges who look at the cases also play into the end results. However, there are guidelines and general rules that can explain why items end up named like they do.
Trademarks are categorized into groups of products/services, so when a company applies for a trademark, lawyers compare them to similar marks within the same category, not across all products ever. Transformers toys fall under International Class 28, "Toys and Sporting Goods."
The name "Bumblebee" provides an illustration of how these categories work: during the time Hasbro was unable to mark a Transformers toy named "Bumblebee" (see "notable cases", below), Hasbro licensee Fun 4 All was able to release a keychain named Bumblebee, because keychains are not categorized as "Toys and Sporting Goods". Similarly, the character of Bumblebee could be referred to by that name within a comic book, since the comic company was not actually claiming it as a trademark within that field. If they were to release a comic series titled "Bumblebee", though, a trademark search and claim for comics would have been needed.
This is also why there can be car batteries with "Megatron" as a brand name, despite having no connection to Hasbro or Transformers. Hasbro does not make car batteries or anything along the lines of automobile parts, so the odds of "brand confusion" are exceedingly remote, making the battery-maker safe from litigation.
Often, companies that own character-based Intellectual Property (IP) will apply for trademarks across a vast swath of categories to maximize the range of product on which they can use that name. Around the time of Robots in Disguise, Konami brought the wildly successful Japanese collectible card game franchise Yu-Gi-Oh! to the US. Among the trademarks they applied for across multiple categories was "Space Megatron," the name of one of the game cards (called simply "Megatron" in Japan). Some fans, upon hearing this, believed that Hasbro was in danger of losing the "Megatron" trademark. However, since Hasbro had made steady and repeated use of the name over the course of the previous decade (over a dozen toys branded “Megatron” between 1990 and 2001, plus media association thanks to five years of the name belonging to a central villain of the associated TV shows), this seems unlikely. The card in question did end up being called "Space Megatron" because at the time that Konami applied for that trademark, Hasbro did not have any Transformers trading or playing cards and thus could not contest the claim in that field. However, no "Megatron" toys were produced in association with the Yu-Gi-Oh brand (ironically, the name was never applied to an ACTUAL card, simply a card featured in one of the early Yu-Gi-Oh video games).
Once a trademark is claimed, legal protection of that mark hinges primarily on usage. A company cannot defend a trademark that they never actually use. Protection of a trademark in the action figure world typically lasts roughly a year, though this (like most anything involving trademark law) is not ironclad. (See "notable cases" below.) The longer and more frequently a company uses a trademark, the stronger their case becomes to retain ownership of it should it be contested by a competitor.
Registered trademarks are a stronger level of protection, thanks to a more involved process (and some more fees), but are not by any means absolute protection. Even a registered trademark can be lost due to lack of use (abandonment) or a successful challenge from another company.
Defensibility is also determined by the name itself and its distinctive qualities regarding to what it’s being applied. One could not reasonably expect to claim trademark of "Apple" as a product name for apples. But for, say, computers, which would not otherwise be referred to as an "apple", that works. In regards to toys and Transformers, "Car" would be basically impossible to defend successfully as a distinct trademark. "Carblast", however, is fairly strong, being a made-up compound word highly unlikely to be used elsewhere. "Autobot Carblast" is even more defensible, thanks to the addition of a clearly nonsensical word, which is by itself very strong, like "Rodimus" or "Cybertron".
Because Hasbro has been shoring up some of their infrequently-used "common word" names in recent years by adding prefixes (such as "Autobot Tracks" or "Decepticon Frenzy"), fans sometimes mistake this to mean that reclaiming a trademark from another company is as simple as writing a faction name in front of it. This is not true. If it were, then a competitor such as Bandai could release a line of robot toys called "Megazord Transformers" with an "Optimus Prime Zord" toy and get away with it. Hasbro's addition of extra words to these old names is primarily to strengthen the claim to a name that is currently (likely) uncontested and Hasbro is likely to try and use often in the future. It is also the case that the names Hasbro has reinforced in this way are all real English words, not made-up words. Real words are harder to defend as a trademark than nonsense words. Adding nonsense to a real word makes it more defensible, but adding nonsense to somebody else's active trademark is only asking for trouble.
Based on some of Hasbro's naming choices, it could be argued that the strategy of Hasbro's IP lawyers is somewhat conservative or cautious. They may be able to defend more than they try to defend, but they choose not to take that risk.
Trademarks can also be lost by allowing a term to become a "genericized trademark" -— effectively, letting their mark become a common term, like "aspirin" for acetylsalicylic acid. Aspirin was once a trademarked brand name controlled by Bayer, but in a 1921 court case it was ruled that the name had become genericized and could be used by all manufacturers of acetylsalicylic acid. Note that this applies to the US. Whilst "aspirin" is also a genericized word in the UK, in other countries it is still a protected trademark and other producers of acetylsalicylic acid must use a different term. The status of a trademaked word in one country often has little bearing on its use in other jurisdictions. Other brand names that have become partially genericized (although still protected as trademarks to the extent that competitors cannot apply the name to their own products) include "kleenex" for facial tissues, "jell-o" for processed, gelatinized pig hooves, "xerox" for photocopies, and "google" for computer searches.
Recently, Hasbro has become concerned about the mark "Transformers" becoming genericized, which would allow competitors to use the term "transformer" as a generic term for their shape-shifting robot toys, creating further confusion among parents when asked to buy a Transformer. Hasbro has implemented various methods of preventing this. For example, use of "transform" as a verb when being used to describe product is almost strictly forbidden in certain written materials. Although this mandate does not extend to character dialogue within fiction, synonyms like "convert" are used in Hasbro's own products (like packaging call-outs and instructions) and in licensees' products (such as game guides).