The horse trainer lightly snapped his blacksnake on the ground to keep the horses focused.

blacksnake 是一个复合词，由 black（黑色的）和 snake（蛇）组成。这里指一种黑色的长皮鞭，因其形状细长如蛇而得名。这种鞭子常用于驯马或驱赶牲畜。