From MHWiki

I think that Monster should be highlight as rare mouse in moz.. --LondiBoy 14:44, 13 June 2009 (UTC)

According to the wiki guidelines a mouse is marked rare if the chance of encountering it is less than 1 in 100 (1%). The monster mouse is encountered around 1.4% see Mouse Populations. The monster mouse is hard to catch - but is not considered rare. -- Ralphminer 15:03, 13 June 2009 (UTC)