it's just a notation...
Usually this is how it is named as vertical or horizontal polarized....
If the antenna E-field is perpendicular to the ground, it is said vertical polarization...
if the antenna E-field is parallel to the ground, it is said horizontal polarization...
Since it's just a notation, you can choose what ever way as the horizontal or vertical... IF the antenna is rotated, polarization changes... just two names given to identify two common uses of polarization