alt Hacker News> And Mermaid could have decided they want exclusive use of the name and trademarked it half a decade ago.
If you do not trademark your name then I can only assume you do not want exclusive control of that name - I certainly wouldn't assume you want the protections a trademark grants.
"You didn't register your trademark, so I guess you don't care if I use it!" Setting aside mermaid here, that line of thinking is not only legally precarious but heavily unethical. In the US at least, trademarks are implicit, like copyright. Registering a trademark can help in court, but it's not required.