It's possible that this word was invented, or perhaps a misspelling of a different term.
Here are some terms that *are* used to describe someone from Halifax:
* Haligonian: This is the most common and widely accepted term.
* Halifaxian: This is another acceptable term, although less common than "Haligonian."
If you came across "Halagonian" in a specific context, please provide more information so we can better understand its origin.