The Collaborative International Dictionary of English v.0.48:

Coachwhip snake \Coach"whip` snake"\ (Zool.)
   A large, slender, harmless snake of the southern United
   States (Masticophis flagelliformis).
   [1913 Webster]

   Note: Its long and tapering tail has the scales so arranged
         and colored as to give it a braided appearance, whence
         the name.
         [1913 Webster]

WordNet (r) 3.0 (2006):

coachwhip snake
    n 1: a whipsnake of southern United States and Mexico; tail
         resembles a braided whip [syn: coachwhip, coachwhip
         snake, Masticophis flagellum]