The Black Stallion and the Lost City — Steven Farley

When Alec and the Black are hired to work as stunt doubles in a film about Alexander and his horse, Bucephalus, they find themselves on set in the remote mountains of the Greek/Bulgarian border. Movie making involves a lot of waiting, so they set out for a morning of exploring. Chasing an elusive albino mare, the two find themselves caught in an underground river which drops them, half-drowned, beside a city lost in time.

Revered at first, they soon discover that they are intended as the entertainment at a horrific ritual . . . sacrifices to the legendary flesh-eating mares in the coloseum of King Diomedes.