1) Bangtail 2) Good wet ground racehorse 3) Horse that does well in slop 4) It runs better in the rain 5) MUD terminology 6) Race horse 7) Racehorse 8) Wet-track runner 
A horse that races well on muddy tracks. Also known as a 'Mudlark'.
[n] - a racehorse that runs well on a muddy racetrack
noun a racehorse that runs well on a muddy racetrack
a racehorse able to perform well on a wet, muddy track. · an athlete who performs well in muddy conditions.