tail of a swift horse; behind a great person
Rare literary noun, almost exclusively used in the set phrase 驥尾に付す (to follow in the footsteps of a great person). The figurative sense derives from the idea of attaching oneself to the tail of a fine horse to be carried along.
See also: 驥尾に付す
The expression 驥尾に付す means to follow behind an outstanding person and advance oneself.