I posted this in the Japanese stack exchange but the word appears to be made up based on early feedback and google translate.
https://wikizilla.org/wiki/Matango#Name indicates this is a variation of the name of the (fake?) fungus but I wonder if any character uses the term -- I do not recall this happening but it has been many years since I have watched it.
I like the suggestion from Adamant that the word comes from Spanish "killer fungus."
In the Japanese stack exchange someone said the name came from a real mushroom but I think that even if true, the "killer fungus" idea is very compelling -- the name of the fungus in Japanese is "mamadango" not so close -- I suggested that matango is a sort of pun: https://japanese.stackexchange.com/questions/97712/literal-meaning-of-%e3%83%9e%e3%82%bf%e3%83%b3%e3%82%b4/97718#97718
