Mass noun is the term for that. Octopus is generally only a mass noun if talking about their meat. As far as the plural goes, it is just a weird corner of the

Mass noun is the term for that. Octopus is generally only a mass noun if talking about their meat. As far as the plural goes, it is just a weird corner of the language where there is no consensus on what the right word is. merriam-webster lists all three variants as plurals.