Why was Dick Turpin's horse called Black Bess?

it wasn't called black bess at all he never had a named horse because he rode stolen horses black bess is a name people got from the poem the highwayman because they assumed that the poem was about dick turpin and in the poem he has a girlfriend called bess who had black hair so a leganed surronded him that he called his horse black bess after her but he didnt. the poem the highwayman may not even be about dick turpin