I asked a friend who knows an IP lawyer about this and hopefully he'll get back to me with clarification.
Generally, "common word" is a very VERY simplified explanation. An explanation from the lawyer is "cluttered market". Some words are used in so many different fields so it's hard to just claim them as trademarks in the "toys" field.
What I gather from this is that some words are just used *too* widely to be easily trademarked, while other words aren't. But... with trademark law there are no easy answers for us layman to understand.
Walky once said with trademark law there is no "1 and 1 equals 2". It's more like "1 and 1 equals 2.7".
I'll try to get an answer for this too. According to Hasbro, they had planned him as Octane, but ended up having to call him Tankor. They even hint his original name in his bio.I don't have a problem with them doing that... but I do have a problem with them slapping completely irrelevant names onto a Transformer that's meant to be called something else; e.g.: Universe Tankor. I understand that Hasbro may not be able/willing to use the name "Octane" on its own, but why not modify it? "Decepticon Octane" or hell, even "Mega-Octane" (which Hasbro used in RiD) would've been infinitely preferable than call it freakin' "Tankor." (-_-)
I don't have a problem with names like "Autobot Jazz," "Decepticon Frenzy" or "Shockblast"... I can even accept "Hardshell"... but "Tankor" instead of "Octane"?? They're just not trying...
It is unlikely that Hasbro ditched Octane to protect Tankor as some have suggested, because if they wanted to protect that trademark they could just have easily slapped it on any other toy rather than knowingly give a toy the wrong name. Basically, there must have been some problem that prevented them from calling him some form of Octane.
I don't believe Hasbro is unwilling to use "bomb" in a toy's name, since back in 2005 made a female GI Joe figure from the comics that would have been named Bombshell, but since that trademark was unavailable, they renamed her Bombstrike.