This usage of "хоть бы" is not a feature of modern Russian.
The full form is "хоть бы и самому ..." and it means "it would be appropriate even for ... himself":

Хоть бы и самому царю Ивану Васильевичу прокатиться! = "[These horses are so excellent that] it would be appropriate even for the Tsar Ivan Vasilyevich himself to ride on them".