Presumably it's not beetroot then...?
Otherwise, what about sugar beet? This is also the root of a beet plant... In fact, it's just a large white beetroot.
Beetroot, sugar beet, Swiss and ruby chard, and spinach beet are all the same species.
Kohl rabi is a cabbage with a swollen stem -- in fact technically not a root. Not sure what rabi means, but kohl is cabbage, as in Chancellor.
Not sure what a mangel wurzel is -- is it the same as a swede? If so, it's the root version of seed rape (canola in US). Pretty sure it's cabbage family though, not a beet.